Usual snowfall on the forums will be good.
I was thinking of a system that someone (High staff member) Would be santa claus and reward the good guys for doing a good job all year or completing a certain quest. Linking to this idea , we could do a little treasure hunt on the servers or do contests for each server which can all be different due to the different mods, but related to christmas.
With the MC server, if we finish setting it up on time, we can make a christmas theme for players and add a whole new quest just for Xmas on the server, along with all servers.
A forum related contest can be a christmas carol contest, where everyone writes a christmas carol and the best will be given an award or whatever you like.
Santa hat on the logo in the top left corner would be cool and other adjustments to the forum will help create the holiday mood

I will think of some later, atm i am tired.