The Bundestag and the Brandenburg gate are right beside each other - you can get up to the glass dome for free but there's usually a half hour queue. The Holocaust memorial is also nearby. Berlin zoo is one of the biggest in the world and has lots of different species, if you're into that sort of thing. You could spend the day there and not see allof it. Restaurant in the zoo is not great though (canteen food), and while the zoo is fine, the area near the Zoo Station is a bit dodge.