There is a FANTASTIC shop called KADAWE that has a food court where you can get a meal made up in front of you - the chinese stand has duck to die for!!
The zoo is nice - you can see Knut the polar bear [not as cute and cuddly now I might add!!].
For something different - you can tour an underground nuclear bunker from the Berlin Story [get a combined ticket for the museum which charts the history of Berling...more interesting than it sounds].
A trip out to the Olympic Stadium is interesting as well as it's one of the few places where you can see Nazi architecture.
Talking of that - checkpoint Charlie always attracts crowds. You can get your passport stamped with Russian, British etc stamps which will always be a reminder of your visit.
Have a great time
