Stay in the East Side of town around Mitte. Do the 'Famous walking tour' with insidertour.com and see if the tour guide 'Brian' is around. Eat and drink plenty of curry wurst, potato, saurkraut and unhomogenized beer. Base your orientation around Alexandre Platz. Explore area around Hackerscher Markt.
Check this site out
http://cafekulturberlin.blogspot.com/
Ask questions also here under the Berlin forum :
http://www.toytowngermany.com/forum/
Avoid Kurfürstendamm (West Berlin famous shopping street) unless you want the Champs Elysee
experience i.e. standard chain stores and fastfood restaurants you can find in every major city of the world . E.g H&M, Virgin, MacDonalds, GAP and etc. You also get massive crowds
The last thing you should do on the morning you leave is go up the TV tower. Nicely puts everything into perspective.
Berlin is very flat and and can be easily walked and/or cycled or you can jump onto their extensive metro, Tram, and rail network. Take it easy and enjoy just being there.