In reply to rallymania:
I was in a similar situation a couple of years ago and had a day or so too look around by myself before doing work stuff. I was on foot and was able to walk around most of the stuff that is more centrally located. If you are used to long crag walk-ins you'll be fine.
My hotel was towards the north-east of the city so I started wondering towards the Berliner Dom (cathedral), had a look round that then headed towards Checkpoint Charlie, there are several more churches and political buildings with impressive architecture between the cathedral and then checkpoint. There was an interesting museum thing near check-point Charlie that gave a good idea of what things were like when the wall was there.
Then you can wonder west a bit to the Topographie des Terrors, which is a long section of the wall on which they have documented a history of the Nazi atrocities and the events leading up to them (its free). One thing that really stuck me about berlin is how humble and open they are about their past, they don't try and pretend these events didn't happen and hide them away, everything as out in the open as a lesson to future generations.
The you can wonder north towards the Jewish Memorial which is well worth looking around, and I would highly recommend going into the visits centre beneath the memorial. Don't expect to come out of it laughing and smiling, but it was incredibly moving and thought provoking. One of the exhibits consisted of letters written by Jews in the camps to their families and these had me (a grown man) crying my eyes out.
After that you are pretty close to Brandenburg Gate and the Reichstag (I didn't have tome to go in unfortunately).
Another thing that was highly entertaining was the 'trabby tours', where you dive around in a convoy of Trabants (the old East German cars) and a guide tells you about the sights over the radio. I cannot describe how bad these cars are to drive but they are also hilarious and a novel way to see the city.
Enjoy your trip. Would be interested to her what you end up getting up to.