We went to an old PoW camp. It was really interesting and I'm not sure if Germany/Berlin advertise it's existence.
We took a train on about a 30-45 min journey out of Berlin (I think) and then a local bus.
It's called Sachsenhausen, it was really interesting, if a bit eery. As we got off the bus outside of the camp an old guy asked if we'd like to be shown around. For a Euro each (there was 4 of us) he took us round and explained the whole thing to us. He had had family who'd been in the camp and told us the facts with compassion and interest, it was so hot 90 deg+ and he was in his 80's, he was amazing.
http://en.wikipedia.org/wiki/Sachsenhausen_con centration_camp
Go to Savignyplatz S-Bahn station, not loads there, but it is a really good & nice 'local' area. Nice restaurants and bars with a local buzz. We went to a pizza restaurant/bar just by the train station where we were served a HUGE pizza. Was honestly big enough to serve 10, each slice was about 12 inches+ long!!! A great experience, not one you get everywhere. But there are loads of other bars too, plus the famous trendy homeware store/centre, which is an interesting place to look around if you are into that type of thing.
In all Berlin is a fab city, I loved it. I spent 2 weeks there last year during the World Cup. Fab place, lovely people, great buzz. Enjoy yourself!