I'll be there in Dec as well for 3 days for sightseeing and relaxation and I'll also be staying at the Hilton. I run here but I didn't think about running there even though Tiergarten would seem to be idea. This is my first trip to Berlin but I'm developing an idea of things to see based on the NY Times travel section and Frommers. I posted a link another thread. Also there are tours like Berlin Walks that are only about 3 or 4 hours but you can hit a number of the major attractions. After that I just plan to walk around and check things out.