The parks,museums and galleries around the Charlottenburg area are lovely and not exactly tourist central
http://commons.wikime...in-charlottenburg.jpg
The bus system is very easy to understand and reliable - get a street map with the bus routes on and go for it.
Least good use of time: queueing to go up the TV Tower in Alexanderplatz - spend time instead looking around the Nikolaiviertel which is quite nearby, and one of the only 'old' bits of the city to survive - lots of shops, courtyards and places to eat.
Around Adenauerplatz we found loads of really good eateries, especially Xantener Eck - but its such a great city you can't really go wrong.