The Reichstag is probably pretty cool this time of year. . . .When my husband and I did it back in January, we weren't really impressed, but that was after waiting in line for an hour when it was 10F outside. . . .
I would also recommend the Kaiser Wilhelm Memorial Church. It was heavily damaged during WWII and never repaired. Its pretty interesting to see. . . .The Jewish Memorial is really amazing, as VictoriaS said. . . .And, if you're much of a shopper, you should stop by KaDeWe (I think its one of the biggest dept. stores in Europe.) They have a pretty good interntational food section, where my husband and I were able to find all sorts of American goodies. (Including Easy Cheese!)
Please post and tell us how your trip was. I'll be back there in a few months, so I'd love to hear about things that haven't been mentioned already!