Amsterdam: Talking about the Netherlands, the first thing to mention is Amsterdam. Amsterdam is really worth seeing ! But I have to add: if you know Asmterdam you don't have a clue about the Netherlands. Amsterdam is a universe in itself.
Of course, you should see the Red Light District and visit the millions of coffeeshops :-). I would start a trip to Amsterdam with a boat tour on the waterways, the "Grachten". In most cases, you will arrive at the central station, walking up to the town center on the Damrak, the boats are the first thing you will see. The boat trip gives you a good impression of Amsterdam and its history. Besides of a couple of interesting "serious" museums (Van Gogh Museum, Museum of History, Rijksmuseum, Jewish Museum, Nautic Museum), there are also lots of bizzare museums (two Erotic Museums, Torture Museum, Cannabis Museum) which are lots of fun. You have to visit the house of Anne Frank, but you should be there at least half an hour before openening, since during daytime, the line of people reaches down the street. But it's worth queueing ! Nice, but unfortunately quite expensive is Madame Tussaud's at the Dam. Our "secret hint" for everybody who loves fantasy art etc.: you should't miss "Chimera". You'll find the little shop at Damstraat, the street leading from the Dam, the "Main Square" to the Red Light District. For shopping addicts, there is a beautiful shopping mall just at the back of the king's palace in Nieuwe Zijds Voorburgwal. But I would recommend it not only for shopping - the architecture of this building is marvellous ! If you like animals: visit Artis, the Zoo of Amsterdam, you should't miss it, it's great (they even have manatis !). If you don't know where to go in the evening, go to the Nationaal Monument on Dam. There, the whole town is gathering in the evening and for sure you'll get a hint where to go!

Groningen: Of course, I also want to mention my "home town" in the Netherlands. Groningen is a University town, so it's crowded with students during the university year. There are millions of pubs, lots of parcs and green areas and they have a nice shopping street. In every case - it's worth a visit!

Utrecht: In my opinion, Utrecht is the most beautiful town of the Netherlands. It's a medieval town with a funny church (the Domkerk): the tower and the rest of the church are separated since the middle part of the church was destroyed by a hurricane in 1674.. At the waterways, there are footpaths with restaurants and bars. If you are romantic, you should go for a walk there in the evening !

Rotterdam: Maybe not the most beautiful Dutch town, but interesting and impressive. You have to do a tour with the boat around the harbour. It's unbelievable, you can go for hours without reaching open sea ! The container ships are impressive ! Next, you should visit the cubic houses, they are fun (althought I always had to think about what the special furniture you need there would cost :-). At the entrance of the harbour, there is a Nautic Museum which is also worth seeing.

Texel: You shouldn't miss this island in the North of Amsterdam, especially if you like south french life style; you'll find it there. On Texel, you'll find sandy beaches and pittoresque little villages. But you should go there at good weathers, when it's raining it's quite boring (which I found out during our work at the oceanographic institute of Texel). There is a ferry from den Helder to Texel. While waiting for the ferry, you could visit the submarine in front of the Marine Museum of Den Helder - it gives you a good impression of the life in submarines !
Afsluitdijk: If you are going by car, you should pass the Afsluitdijk once. It separates Ijsselmeer and the North Sea and is a marvellous piece of engineering. There, you can also watch the highway separating and you'll find traffic lights on the highway: that's for larger ships of the Ijsselmeer - you'll see the unique Dutch bridges!