Bus tour: there is a regular bus line than basically passes lots of important sites like Schloss Bellevue, etc. I believe it's either line 100 or 200 and passes S- and U-Bahn station Zoologischer Garten (right at Kurfürstendamm (the big shopping street)). Of course, on a regular bus line, it's pretty easy to hop on and off, also it doesn't come with a typical tourist rip off price. Just ask at the hotel, they know about this line.

From there, you can still do lots of walking and Berlin is a great city to walk in.

The only thing I really hate are the traffic lights. I am a really fast walker but I can basically never make it across the street on a 2-lane in each direction intersection without seeing the red light when I am almost over. I ask myself what older people or people with other handicaps do in Berlin?

Anyway, have fun, you will have just missed the big party for the reunification.