You think Germany shuts down over Christmas??!! You've obviously never spent Christmas in the UK... which really DOES shut down for Christmas. No public transport on Christmas Day, and hardly any on Boxing Day.

Shops are shut over Christmas in Germany, like they are on all Sundays and public holidays. That's something you very much have to expect in a lot of European countries. Some other attractions will be open on Christmas Day and/or Boxing Day, as others have mentioned. Christmas Eve afternoon/evening is the closest Germany comes to shutting down, as that's when everybody is at home with their families.

If the weather is nice (and I haven't checked the forecast) why not jump on a train and take a trip out into the countryside on Christmas Day or Boxing Day?