Howdy there and welcome to the DIS!! We went last December 7-14 and everyone told us to be prepared for any kind of weather and boy were they right!! It never got very warm any of the days so most of the time we were in pants and long sleeved tops but there are other years where it may get into the 80s. It also rained a lot - which is very unusual for December - but left us unprepared. So plan on packing a little bit of everything!!
For activities - there are so many I'm not sure where to start. We did
MVMCP, the Candlelight processional, they have Christmas everywhere so be sure to check at each park for a list of what is going on that day. It was very uncrowded for us and we still used fastpass for some things but for the most part didn't wait in a lot of lines.
Have a great time - you'll love it!