Groningen is definitely not the largest university city, but it is one of the most popular ones. And I should know as I grew up there.. Due to the large number of students, nightlife is great and most bars stay open until 6AM on weekends.

The medieval center has lots of sights, including the Martini Tower, City Hall and the canals. The Groninger Museum is set in the middle of a canal and the building is considered as one of the most modern and innovative museums in the Netherlands.
Just a 2,5 hour train ride north of Amsterdam...