We splurged on the hotel (after all we didn't have to pay for flights or a rental car!) and stayed at the Adlon Kempinski. Plenty to see and do: Brandenburg Gate, Holocaust memorial (both close to the Adlon), Reichstag (prepare to wait to get in unless you make a reservation for lunch), Tiergarten, the Berlin Wall remains and exhibit on Bernauerstrasse and other places, the Rittersport chocolate shop(!), the Olympic stadium, Checkpoint Charlie (which was to be frank very disappointing, cheesy and commercialized), KDV - the oldest and best department store, Berlin's equivalent of Harrod's.
Make sure you have good walking shoes unless you want to use the U-bahn (subway) or the S-bahn (overground). Rather than pay for extortionate hotel breakfasts we went to Cafe Einstein, a few blocks from the hotel, every day - traditional German breakfasts. My wife speaks fluent German and on our fourth visit we were shown to the back of the Cafe which we think was (secretly) reserved for locals - proper tablecloths etc! Also for dinner the Adlon has a great Thai fusion restaurant - Sra Bua. We also ate a couple of evenings at Ständige Vetretung to satisfy my wife's cravings for German food and it was very good, but be prepared to wait if you get there after about 7:00.
I would recommend buying a good guidebook too - there is plenty more to see.
If you go, enjoy, it's a great city. We also asked in advance (called Babara at the Volvo Baltimore reception center) for them to install European maps in the GPS which made navigating much easier. They said they would install US maps before the car is shipped to the US.
If you have any more questions, please ask.
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