Groningen is also a great city to check out. The city's population mainly consists of students so there is a very active nightlife. There is also plenty of sailing opportunities in the north if that is your cup of tea.
As for the rest of the country, Utrecht and Rotterdam are quite fun if you know the right places. Utrecht looks a little like Amsterdam but has a lot less tourists, this means you can see and do a lot of the same stuff you would be able to in Amsterdam but for less money and while being surrounded by Dutch people. Rotterdam on the other hand was almost completely bombed in WWII so has a lot of modern architecture and ditto museums.
Regarding Amsterdam, the Rijksmuseum just opened (use the google) and it is supposed to be pretty epic, on par with the Louvre and what not, prepare to queu though.
If you have any specific questions you can always ask them here or PM me. For what is worth I live in Nijmegen which is near the German border, would be happy to grab a beer if you are in town.