Originally Posted by
dingo
Antwerp has been mentioned, but Brugge or Brussels is an option as well. I'd go w/ Brugge in a heartbeat over either of the other mentioned Belgian cities. Brugge is delightful and an easy train from Amsterdam.
I'd second that. Bruges (in French, and usually English) or Brugge (in Flemish) is a wonderful city to visit.
For something a little different you could go to Luxembourg: not a long trip. It's an interesting place, not a "party city" I suppose but there are things to see there and it's very scenic.
Or you could go into France and visit Lille, or Reims, or even Paris...