If I could recommend only one thing from my time in Berlin, this would be it!
It is probably the most amazing art exhibit I have ever seen. Christian Boros and his wife bought this WWII bunker to display their private collection of contemporary Berlin-based artists. Each piece of art is impeccably displayed in concert with the building itself and the other works surrounding it. They are currently closed to install the second set of their collection, but willbe reopening on September 2012. The gallery can be seen only with reservation for a private tour (only 10 euro) in English or German. You can find more information and make an appointment www.sammlung-boros.de