Harold gives great advice. You have to know your interests. Then you can make a plan of how to best use your stay.
The OBW (Original Berlin Walks) tours are a wealth of information and a great introduction to the many periods of German history.
Each visit I have done one of their walking tours and it added depth to my visit along with the fact they fit a low budget.
I don't know your budget. Perhaps you may like the the Holiday Inn Berlin Centre Alexanderplatz. You can reach the hotel by the bus (TXL). Breakfast is 15 euros but you can find other places just around the corner and the immediate area of the hotel for good value.
Address: Thea Nolte-Baehnisch-Strasse 2, 10178 Berlin, Germany
Phone Number: 0049/30/7407470
For 1 night on the lower end there is the Ibis Budget Berlin Alexanderplatz, Mollstrasse 30, 10249 Berlin,
Phone Number: 0049/30/339391120
Breakfast place: Cafe Einstein; coffee/espresso, cake, yogurt/yogurt w-fruit or fruit salad is like 11 euros.