When our kids were little, we came up with the idea of sleeping under the Christmas tree. It became a yearly tradition. We'd haul out a big mattress and fill it with comforters and pillows, sliding it as close to the decorated tree as possible. Everyone would put on their pajamas, then we'd cook up a pizza, load a movie (almost always A Christmas Story), and settle in for a fun night. The best part, of course, was after the kids fell asleep and you could watch their little faces glowing in the twinkling lights of the tree, dreaming happy dreams.
If you don't have children, this makes a pretty good "adult" activity too! ;)