Magic Kingdom - See a Xmas wishes fireworks show. These are on during Mickey's Xmas Party, but if you don't want to spend the extra money, go to the Polynesian hotel on the monorail, walk down through to the beach, and watch the display from there (or book a restaurant that can see the display for the right time that night)
EPCOT - Watch the candlelight processional in America. You'll have to queue for ages but it is very worthwhile. Better still, cut the queue by booking a dinner package.
Elsewhere in World Showcase most/every country has storytellers telling traditional Xmas stories from their country -very cool.
Also watch Illuminations fireworks for the extra 5 mins at the end to celebrate the season. The most incredible 5 mins of fireworks anywhere, ever in my opinion!
Hollywood studios - Osborne family lights. On every night from about 6pm. Breathtaking.
AK - Not that much Xmassy stuff here, but enjoy the lighter queues for the rides and don't miss the Lion King show.
Resorts - each resort has its own decorations which are all outstanding. Make sure you visit the Grand Floridian where there is a huge real gingerbread house (people work from it as a sweet shop kind of thing) with a sign that explains how they make it each year - I didn't realise at first it was really made of baked goods! GF also has an enormous Xmas tree.
More generally just walk around and soak up the Xmas music and atmosphere. Notice how the decorations are just gorgeous and eat as much gingerbread as you can stand!
I LOVE Xmas at WDW, hope you enjoy your Dec trip!
Miranda