Hi Paul,
You are in for a treat! Christmas at Disney is extra special. I was there in 2003 and it was the best holiday ever.
Points to note......
It will be VERY busy. My experience was that on Dec 25/26 Magic Kingdom closed at around 10am as it was filled to capacity. We spent the day at Animal Kingdom instead which was something a little different. I would avoid MK on the 25th as it can be quite miserable being crammed in like sardines. You have plenty of other days to go there anyway. As you will be driving, for the big days like 24th-26th, New Year etc, you MUST get to the parks early. Leave very early as the queues to get parked will hold you up. I was amazed by the amount of traffic.
In all the parks, have a basic plan of attack. Use fastpass whenever possible and don't really plan on doing EVERYTHING. There are many seasonal activities and we spent a lot of time seeing those instead. World Showcase Stories around the world is excellent. Each country has a storyteller doing a floor show based on traditions and customs, ie UK - Father Christmas, France - Pere Noel etc.
Here is a link to some stuff happening.
http://allearsnet.com/tp/holiday.htm
I would highly recommend the EPCOT Candlelight Processional. It was the highlight of my whole trip. We went on Christmas Eve and it was amazing. We did the dinner package as it's so much easier to get seating in the theatre then, and you don't have to queue up for as long.
Oh, we also went on a sleigh/carriage ride at the Wilderness Lodge resort. It's not actually a sleigh but it is done up to look and sound like one.
Weather - can be variable in December. We had temps of 80 during the day but I have heard others say that it has been cold enough for coats and gloves! I would check the forecast nearer the time and take a variety of clothing.
You'll see all our activities were in the parks as we stayed onsite. As you're offsite, there will be other options open to you and maybe someone else will be able to help with that.
I'm sure I'll think of more things as they come to me. Any questions, fire away!
Louise