I made wee origami Santas. Each guy is folded from two 1 1/2" squares of red paper. Have the kids tuck one inside your Christmas cards or stick them onto gift tags. Make them green and they could be elves.
Learn how to make these easy origami Santas right here at HoikuAkira. The instructions are in Japanese, but I just followed the photos. Google translate didn't add much value; the pictures were enough to get the job done.