Berlin!

As of December 25th Eric the roommate and I are going to Berlin! We're staying in Mitte with two Swedish girls we met on couchsurfing.com and stayed with us last summer.
I've been doing some reading and catching up on what I should hit in terms of museums, food and cityscapes but I could use a tip or two on places where I could get the nitty gritty of the city?

Suggestions?