How about stapling two 1/2 sheets of red/pink, etc construction paper together with one side open like a pocket and letting them decorate the sides for receiving valentines?
(You could also just use a hole punch around the edges and let them "sew" some yarn through to hold them together)
Or.. cut med size hearts from felt or paper and let them glue or stick on Valentine shapes- Hobby Lobby/Michaels, etc has pre-cut foam and felt stickies or you could just use stickers
Or... cut a piece of construction paper into strips and decorate them, then tape or staple to size of their head for headbands.. you can take another piece and cut into thinner strips and fold like an accordian and glue/tape/staple to headband and then hearts to the other ends and make antennae.
Or- decorate brown lunch bags for receiving Valentines
If there aren't any food allergies, you could take white frosted cupcakes and sprinkles, etc to decorate (or let them frost their own plain ones.... or do the same with cookies)
Or.. cookie cutters and clay that you can bake and harden for ornaments. There is a good recipe for applesauce/cinnamon clay that smells good on google.com if you search... poke a hole before baking for a ribbon to tie...