there was a really weird little club/bar called "dr. pong"*when i was there that was a lot of fun. very berlin, in that it was nearly impossible to find, had a tiny sign and required you knocking on various doors once inside to actually find the place.
if i remember right, you would pay a small fee to play a ping pong tournament on a large circular table with many people simultaneously and the winner got crowned Dr. Pong for the night.
also, go to the fleamarket in treptow if it's still going. it's huge and sells everything from tractor engines to soviet army fatigues. come to think of it, there's another hidden cafe there too; look for a concrete bunker near the stream and go down the stairs to the water, there's an unmarked door to get in.