FWIW, here is my general guide for determining what to do on Christmas Day.
1) If you want to experience the "magic" on Christmas and go to MK that is okay. But understand you will not go on many rides, will not get a good view of the parade, will have long waits for meals, etc. MK usually closes to new guests by mid-morning and is wall-to-wall people all day long. As long as you understand that you cannot do much and are happy with just soaking up the atmosphere, then you'll be fine.
2) If you have more than 6 days at Disney, take the day off from all parks. Go resort hopping, check out DTD, swim at the resort (if warm enough), or do some of the other activities people have already suggested. Unless you have an UPH, make a PS for meals at a non-park restaurant. If you have an UPH you can make a PS to eat anywhere, including a park restaurant. But if you eat in a park I recommned just going there to eat and walk around. Don't try to do any rides, etc.--save that for another day).
3) If you only have a few days at Disney (<6 days) you probably can't afford to take a day off. In that case, go to Epcot on Christmas. While it is crowded, it is less crowded than the other parks and can handle the crowds better. Plus, there are fewer children, as they tend to like the other parks better. Get there early if you want to go on the big attractions (M:S, TT, etc).
4) No matter what you do, make a PS for meals, especially dinner.
These are just my suggestions and I'm sure others have their ownideas and will make their suggestions.