Basically get away from the casino area as it is the most sterile and boring part of Berlin
Sunday - go and sing or watch Karaoke at Mauer Park flea market
Tues/Fri - Visit the Turkish Market at Maybachufer for great Turkish food, nice atmosphere
Tempelhofer Feld - Former airport that is now a park, you can rollerblade down the runway
Design Panoptikon/Museum of Industrial Objects - Torstraße 201 - Crazy Russian Vlad will guide you through his private museum of random objects from all around the world, arranged into displays. You have to guess what they are and hear their history.
Spreepark - Break into a former East German theme park, take pictures and look at the rides and don't get caught by the guards. Best visit at 5am!
Undergound Tours - Berlin Unterwelten - tours through former bunkers, nazi anti aircraft towers etc.
The monster kabinett! -
http://www.monsterkabinett.de/ - caution sound on website!
I might be able to hook you up with some private cash games if you private message me!