Congratulations! You're going to have a great trip. For me, planning is the key. We get a lot done but it means early mornings and sometimes later evenings.
This year will be, I believe, our 4th trip to WDW during Christmas week. We have always done EPCOT on Christmas Eve. We love walking around WS and enjoying the festive spirit. For dinner we always go to the Biergarten. It is my favorite restaurant and very festive. It helps that I do all of the planning and make all reservations. We also do the Candlelight Processional on Christmas Eve.
On Christmas Day we go to MK. We understand that there will be massive crowds as the day progresses, but with planning it isn't bad at all. We get there when the park opens and enjoy the atmosphere. We ride what we want to until about noon (when the crowds get really big). At this point we'll have lunch (CP this year). After lunch we enjoy the parades and other non-ride attractions for a while. In the early evening, many people exit the park so it becomes less crowded. We usually save our FP's for the evening and just go with the flow. This year we are doing the desert party so we won't have to fight the crowds for the fireworks.
We take a small pre-lit tree with us. I think it's 3 or 4 feet. I think I got it at
Walmart for $20. We take some of the easy / smaller gifts and either pictures of the tree at home with other gifts or a note. They also have these cool light up snow flakes to place on the windows to alert Santa that you are there. I can't remember of we bought them at WDW or Target or something. When the kids were younger, they really enjoyed that.
On our last Christmas trip we went to one of the water parks. It was chilly, but we're from western New York so it wasn't too bad. Sort of neat to be doing that in December. We also do lots of resort hopping and spend 1/2 a day at Disney Springs.
Have fun planning!