Lunch in the Kafer restaurant on top of the Reichstag (must book in advance though). Amazing view and good food.
DDR Museum by the river, very interesting about everyday life.
Open top bus tour, great way of seeing all the things you otherwise arent that interested in.
Food wise visit a Block House and Allegretto by the river do a smashing Curry Wurst.
I didnt get the chance but the Trabi safari looks fun too!