You're going in December which is a great time to go because of the additional decorations & entertainment for Christmas. Going on a Thursday in early December will be good, the crowds should be low. One piece of advanced planning that you may want to do, is reserve a candlelight processional dinner package. Read more about it here:
http://www.allearsnet.com/tp/h_cpfaq.htm
The information on that page is from 2003 but it will give you an idea of what it's like. The 2004 information should be released pretty soon so keep your eyes open for that.
OK, as for your day at Epcot, start out in Future World, going straight to Test Track first thing, followed immediately by Mission: Space. (It's right next door.) These are the two most popular attractions at Epcot and get crowded very early in the day. After this, head over to Spaceship Earth (the big geosphere) and ride that. After this then work your way through the rest of the Pavillions of Future World.
By early afternoon or by the time you're ready for lunch, make your way over to the World Showcase. Maybe eat lunch at one of the countries' counter service restaurants. Basically the afternoon should be for browsing through the different countries of World Showcase. Some countries have a ride, some have a film, all of them have shops. Some of the attractions that I recommend are the Maelstrom boat ride (in Norway), the American Adventure (in USA of course), & the Impressions de France film (in France of course). I also highly recommend the french bakery.
Also, as it will be the Christmas season, each country will have their own version of Santa Claus giving presentations throughout the day of how that country celebrates Christmas.
The thing about the World Showcase is that it is meant to be browsed through at a slow pace so that you can take in what each country has to offer. It's not just about rides. However, it is very easy to spend an entire day going through the World Showcase, so pace yourself such that you can make it all the way around.
Now as for the evening, assuming you get a candlelight processional dinner package, you will be eating dinner at one of the World Showcase sit down restaurants and then going to the Candlelight Processional (or vice versa). Be sure that you arrive at the theater (for the CP) about 45 minutes ahead of time.
The end of the day is topped off with Illuminations, a fantastic fireworks/music show that takes place on the World Showcase lagoon. I think the time for that will be at 9:30. If you can try to get a spot about 45 minutes ahead of time. There are good locations all around the lagoon, but my personal favorite spot is over by the Mexico pavillion, just to the left of the Margarita stand.
Have a great time at Epcot! It's my favorite park!
