Back in the fifties, a group of directors and actors led by Branko Gavella separated from the Croatian National Theater ensemble and founded Zagreb city drama theater Gavella. It has been one of the most recognized theatrical institutions of Zagreb ever since.
You can't really find a reason to see a theater play in Croatian? Then having a drink in the Gavella bar is a good idea. It'll take you a step closer to the world of Croatian actors and theater employees. There's a good chance you'll be able to enter the part of the bar that should be reserved for ensemble and staff members only.
Just for the record, this location revealed an important prehistoric founding - some woolly mammoth bones.