Make Christmas tree ornaments from folded paper or old book pages. Add a piece of paper for a trunk. Punch a hole at the top for adding twine or ribbon for hanging. There are many printables you can use, some are Christmas songs - would make a more festive look.
Paper Trees Friendly Forest Think Spring by psitsinthedetails, $12.50
Handmade trees would be a great addition to a card. But they look good to me as fans on a hot day at an outdoor party.
Have the kids each paint several pieces of paper using fingers, corks, combs, etc. using different colors of green, blue, and yellow. Let them mix as they paint. Then after they dry cut them into smaller pieces and share/trade with classmates to create trees. Use a punch to create ormaments.