I know you've got plenty of recommendations already, but I'll go ahead and put my 2 cents in...
1- All Parades and fireworks shows, including Fantasmic. I'll qualify this by saying I have not seen the new parades, but Tapestry of Nations was great, with awesome music. Fantasmic is a big must see. If you can, do a dinner package so you don't have to wait as long for the show.
2- World Showcase at Epcot, including the boat rides in Mexico and Norway, and The American Adventure.
3- Rock n Roller Coaster, Dinosaur, The three Mountains at MK, and Tower of Terror. (Assuming your children are up for these).
4- Peter Pan's Flight.
5- As I'm making this list, I'm realizing that just about everything is awesome to us, so you pretty much can't miss!
Take some time to walk down Main Street at MK, both at day and at night. Listen to the music playing throughout the parks. Look at the lights in various areas, which should be great at Christmas time.
Just let it soak in...