Here are a few good sites for birding in the Netherlands - most of them sites I visit regularly. The first four are all in the general vicinity of Rotterdam.
Maasvlakte Oostvoorne Carnisse Grienden Hellegatsplein / Ooltgensplaat A Delta Route Flevoland Ijmuiden Starreplas
This is a large area of reclaimed land jutting into the North Sea, west of Rotterdam. It was built for industry, but much of the area was not used until recently. Unfortunately much of what was open is now being built on, but there are still some tundra-like open areas. In 2010 work started on a huge further westward expansion, so the situation changes from month to month.
With the Maasvlakte, you never really know what you're going to find. On one visit you might find Tawny Pipit, Dotterel (which both turn up fairly regularly in autumn) and dozens of other species. On the next you might walk for three hours and find almost nothing. The best period is during autumn migration; spring migration can also be good.
Looking at the map you would expect the Maasvlakte to be really good for sea-watching, but I've never had a great deal of success with it there. But by all means give it a try.
Directions: finding the Maasvlakte itself is dead easy: head westwards on the A15 motorway (which becomes the N15 after a while) and just keep going. Detailed directions on the Maasvlakte itself are tricky, because the road layout keeps changing as new bits get built on. It's basically just a question of exploring a bit - good birds may turn up anywhere. In weekends during the migration season, you are likely to find other birders anyway. Don't worry about getting lost - the main road back towards Rotterdam is easy to find.
Oostvoorne (pronounced oast-FORN-uh) is immediately south of the Maasvlakte. In fact the main lake and estuary area can be viewed from the Maasvlakte side, but the light is usually better from the Oostvoorne (south) side.
There are four main habitats here. Firstly the lake, which is actually a piece of river which has been closed off. This attracts quite a few duck and grebes (Slavonian Grebes occasionally and the other four species regularly.) Secondly the estuary area, which attracts the usual waders and also a lot of duck. It's best to be here close to high tide, or the birds are too far away to see properly. Thirdly the beach and dam between the two, where Snow Buntings and Shore Larks can frequently be found in winter. Bluethroats breed in the bushes. And finally the wooded area, which stretches quite a long way south from Oostvoorne.
Directions: from the road to the Maasvlakte (see above) the village of Oostvoorne is signposted. Follow the signposts to the village, bear right in the village centre, and drive to the car park by the beach. If you walk up onto the dam which separates the lake from the sea, you have a good overview to get your bearings.
This is my local patch. It isn't one of the Netherlands' major birding sites, but might be convenient for someone visiting Rotterdam who only has a couple of hours to spare. I've seen over 130 species here, but then that has taken a few years.
The best time is spring, when it has breeding Marsh Warblers, Bluethroats and Redstarts among other species. Short-toed Treecreepers and Willow Tits are resident. Marsh Harriers turn up occasionally, and can often be seen on the other side of the river.
Other species I have found here include Little Ringed Plover, Ruff, Water Rail, Spoonbill, Kingfisher, Icterine Warbler (though not recently), Goshawk and Green sandpiper.
The river here is tidal fresh water, and the chance of waders is better at low tide. Tide times are about the same as at Rotterdam.
Directions: take the Barendrecht exit from the A29. Turn right (west) and drive a few hundred metres to a roundabout. Take the exit just over half-way around, and then almost immediately turn left (heading south - Carnisseweg). Turn right at the junction just before the dyke. Follow the road for about a kilometre until you reach a little road going up over the dyke; there is a car park on the other side. The paths are easy to find.
This is my nearly-local patch. (You weren't thinking of trying to pronounce it were you? Oh OK, approximately it is: HELL-uh-chats-pline / OALT-chens-plaht where the "ch" is approximately as in "loch" in each case.)
This area has sprung up, by a mixture of luck and judgement, around one of the dams built as part of the Delta Works. It has become an area of major importance for breeding, moulting and migratory birds.
One of the features is the number of geese which have started to breed here, and even a pair of Bewick's swans has over-summered more than once.
It is also a rich area for rarer visitors. Birds I have seen here myself include Spotted Crake, Whiskered Tern, Caspian Tern, Penduline Tit, Temminck's Stint, Greater Flamingo and Osprey. Unfortunately the best hide is now closed.
Directions: Take the Numansdorp exit (exit 22) from the A29 motorway (south of Rotterdam). Now you have to use minor roads to follow the motorway southwards, which you do as follows. First head in the direction of Numansdorp. About half a kilometer from the motorway, turn right (signposted "Numansdorp Havens"). At the next junction, turn right and then left (still following "Numansdorp Havens"). At the next junction, turn right and then go straight on (not following "Numansdorp Havens" any more). If you aren't now running parallel to the motorway you are lost.
Drive over the enormously long Haringvliet bridge, trying not to be distracted by the bizarre speed bumps. At the end, turn right, and after a few hundred metres there is a small car park on the left (just after the second motorway underpass). Walk up onto the dyke, and there are signposts to the "Kluut" hide. For this hide, the light is best in the morning. The walk to the hide often turns up interesting birds - e.g. Marsh and Icterine warblers in the spring.
You can now drive further along the secondary road, stopping anywhere that looks interesting.
Drive further still to the end and turn left, towards the village of Ooltgensplaat. Just before the start of the village take the road on the left. After a few hundred metres, at the farm by the sharp right bend, look out for Tree Sparrows in spring/summer. Carry on past the cemetery and take the next turning on the left. Just after the sharp right-hand bend there is space to park. Cross the stile (a bit hard to see!) and over the dyke you can pick up a path to the "Stormmeeuw" hide. This hide is one of the most reliable spots for Mediterranean Gull in spring, though it may take some while to spot one.
This is a description of a route round the Delta. The whole route will take a whole day, but you can obviously choose to do bits of it. The description below will be easier to follow if you have a map of the area.
First find your way onto the A29 motorway, heading south from Rotterdam. At the end of the very long bridge over the Hollands Diep, take the exit signposted Zierikzee. At the traffic lights you can turn left, and then take two more left turns almost immediately, to visit the Ooltgensplaat area. (See Hellegatsplein/Ooltgensplaat description).
Return to the main road (the N59) and follow it down to the Grevelingendam. Take the turning onto the Philipsdam (I'm not sure what it's signposted - Tholen, I think - but it's not difficult to find.) It's possible to park some way after the locks and do a long walk along the cycle path over the dam. All sorts of things turn up along here, including Penduline tits (though I've never seen them here myself.)
Drive southwards over the dam to the end. Turn right, and after about 500 metres turn left (signposted Tholen). Turn almost immediately left again on to the minor road which runs parallel with the main road. After a few hundred metres you come to an open area on the left. This is the Rammegors, which was an exceptional birding area; it seems to have declined a bit now, but is still quite good. This side can be good in spring, but is likely to be a bit dried-up and empty in summer. Next follow the minor road further until you come to a sharp left turn onto a very minor road (actually a U-turn), turn right at the end, follow the road up onto the top of the dyke, and park. Here you can walk through the white gates into the Rammegors reserve (there is a board with a map). Climb back up onto the dike and follow it around the lake. Bluethroat and Bittern breed here, Black-winged Stilts have bred here, Spoonbills and Spotted Redshank occur in season, and there could be practically anything!
Return to the Grevelingendam and follow the road out past Zierikzee (a very pretty little town, incidentally). About 8 km to the west (by the hamlet of Moriaanshoofd) there are some little roads to the left, any of which take you to a road overlooking a couple of brackish inlets on the south, and a fresh-water pond on the north. These can also be very good, especially at high tide.
On the other (north) side of the main road you come to the Prunjepolder, which has recently been flooded and turned into a nature reserve. Large numbers of waders gather here, and it has already produced vagrants in the form of Long-billed Dowitcher and Buff-breasted Sandpiper. There are several places around the polder where it is possible to stop.
Then you can head a little further westwards and pick up the N57 road northwards over the dams.
The Brouwersdam has three parallel roads running over it: a main road in the middle and minor roads on either side. A couple of linking roads connect the two minor roads, so it is possible to explore both sides. In winter, a particularly good stop is on the sea side of the dam, by the sluice gates near the south end. There can be large numbers of divers here, and it is also a good spot for Long-tailed Duck.
At the south end of the next dam northwards, the Haringvlietdam, is Stellendam. The outer harbour can be very good in the cold half of the year. In winter there are often sea duck, either in the harbour or just outside, while on the west side of the harbour wall there are many waders when the tide is not too high.
If you still have time over, you can also visit Oostvoorne.
Flevoland is, after the Waddenzee and the Delta, probably the third most important area for birds in the country. It can be worth a visit at any time of year. It is worth getting a map for exploring the area, but the following route description takes in several of the most important spots.
Coming from Amsterdam, take the Muiderzand exit (number 2) from the A6 motorway. Head northwards along the coast of Flevoland. This road continues along the coast as far as Lelystad. There are several parking spots along the road, almost any of which can turn up something. Note in particular two or three small parking areas on the inland side of the road (i.e. right-hand side heading in this direction) - around kilometer posts 10 to 13. This is the Lepelaarsplassen reserve.
Somewhat further you come to a large lake - around 10 km long - which is the Oostvaardersplassen reserve. Birders sometimes stop on the verge along here, but one isn't supposed to, and with the increasing amount of traffic it is becoming less safe to do so. The place to be is the Knardijk - the first road to the right after the lake. This is a quiet road where you can park more or less anywhere on the verge. After a couple of kilometers there are marked parking places on the right and a path to a hide. The path to the hide can actually be better than the hide itself though.
On the other (east) side of the Knardijk is an area of woodland that is worth exploring in spring. Further down the Knardijk, there is another hide and a visitors centre just before the railway.
Just after the railway, you can turn right onto the Praamweg. Look out for a car park and half-hidden small hillock on the right after a kilometer or two. The view from the hillock (over the railway) can be very productive if you have a telescope. A much larger hillock further along, where the road climbs up to a bridge, is also sometimes worth a stop.
Return the same way, or find you way back across Flevoland, or head for the Veluwe woods on the other side of Harderwijk.
If you have a car, the south pier at Ijmuiden (due west of Amsterdam), and the newly created lake next to it, can be very good in autumn and winter. Get a map, because it's not easy to describe the route to the pier, but it's obvious enough when you're there - the pier is about 3 km long.
An enormous sign tells you that it is strictly forbidden to walk out onto the pier. This sign is completely ignored by hundreds of fisherman, birders and visitors in general. Do however treat the conditions with due respect when there is a strong wind blowing, and particularly if it is increasing: people have been washed away here.
A good spot near The Hague is the Starreplas - a lake between The Hague and Leiden. Take the Leidschendam turn-off from the A4 motorway. Follow signs for Leidschendam, and then for Leidschendam-Zuid. After a few hundred metres turn right when you get to the big canal. (If you end up in the new tunnel running under the canal you missed a turning!) After that it is basically a question of following the road northwards between the motorway and the canal without crossing either (it wiggles a bit).
After a few kilometers you come to the lake. The road turns sharply right along the northern shore and you can usually find somewhere to park there. (Sadly the powers that be have now decided to prohibit parking; however there is a small car-park at the end.) What there is to see is rather variable - depending particularly on the water-level - but usually worthwhile.
There is a rough map of the Starreplas here. The rest of the site gives quite a bit of info, but it's in Dutch.