Berlin is awesome!
Must sees: book the Reichstag early and make a reservation for sundown, this way you can see the movie/lights on the water when the sun goes down. The Pergamon mueseum should be booked with priority tickets if you are going on a weekend to avoid the 3 hr wait times. Be sure to buy a piece of the Berlin wall to bring home, it's a nifty souvenir that can be found by the Reichstag. The place above Hitler's bunker is a great little place to visit although it is now just a playground. Also be sure to check out