Small, but oh my:
Why locals love it
Every Rostocker who is just a tiny bit into art has been here at least once. The Kunsthalle is the only newly built art gallery of the former GDR, which is mirrored in its special architecture. It's not big, but big enough to spend 1 or 2 hours there, and the open bright rooms have proved to be very adaptable for a variety of exhibitions.
Why you should visit it
If you are interested in art, you should definitely have a look what is on in the Kunsthalle. Especially lovers of modern art are likely to find some interesting exhibitions there. In case you are with people who aren't into art, no problem: the museum is surrounded by a park with a fountain, and there is a small café as well.
Admission free, except for special exhibitions. Public guided tours every Thursday at 4pm.