I agree with Phil, I LOVE Dordrecht (though I have to admit I am a little biased as I used to love there). Its not a huge town, but really adorable, lots of little streets, a nice church tower you can climb, some lovely cafes along the river. You can take the fast ferry from Dordt to Rotterdam which is quite a nice experience.
One place I always go is Kinderdijk. Its near alblasserdam (maybe 20 mins from Dordrecht??) and you walk between the dijks in the paddocks and there are well preserved windmills everywhere. I think some of them are still lived in, but other are empty. There are also one or two that you are allowed to go into http://www.kinderdijk.com/
Amsterdam is kind of one of those must-see places, but its not the most amazing place. The Hague is quite nice, there is a model village just outside of The Hague called Madurodam which is pretty cool.
Utrecht is a big-ish city but also really nice, lots of lovely cafes along the canals.
I haven't been but Den Bosch is said to be gorgeous!
Its a bit more out of the way, but down south Veere was also really nice, but very quaint.
In the bigger cities Rotterdam is cool, partly for its history. Its the most modern city in The Netherlands because it got completely flatted by bombs during the war and has been completely re-built. Interesting new architecture there includes the cube houses and the pencil building. (Train station Rotterdam Blaak will get you the closest)
Have a great holiday there!!!
(Sorry my suggestions are all South Holland - thats where my family is, and I regularly visit) almost 7 years ago
Delft is famous for its pottery and is a nice town. Volendam is often coupled with Marken on tours as a place to see. South of Rotterdam if you can get to do a tour of the dyke works it is worth doing. Of course if you're into WW2 history Arnhem is worth a visit, I just haven't got to do the latter yet.
http://www.historylearningsite.co.uk/arnhem.htm almost 7 years ago