It's A Small World Holiday. wonderful makeover of that ride.
Haunted Mansion Holiday. Another great makeover.
Believe...In Holiday Magic. Christmas fireworks with snow as the finale.(my personal favorite)
Christmas Fantasy Parade.
There's also smaller things like the Christmas Carolers, Santa, and children's cookie decorating.
As for crowds, I'm not sure how bad it is on Christmas Day but the week before Christmas through New Years is generally packed to the gills.