I once had a 9 hour layover in MUC (flying ORD-MUC-PEK) and was planning on heading into the city, but the weather was lousy and I had work to do so I stayed in the Senator Lounge. Given the lounge isn't an option, perhaps look into a day room at the Kempinski Airport hotel. It's connected to the airport terminal and I know they have a EUR 99 rate on the weekends/holidays. You may be able to get a special rate for day use and then you can use the facilities (a nice indoor pool and gym) and rest during your layover.

But MUC is a great walking city and will be all dressed up for Christmas, so I'd encourage you to consider checking it out!