Felt ornaments- have them cut out 2 matching shape (freehand or can trace a cookie cutter- a fine sharpie works well for this)- eg 2 Christmas trees.
Either glue together (use felt glue, Elmer's won't work!) or sew with embroidery thread. Include a ribbon loop for hanging. They can decorate with glitter, sequins, yarn, etc. You can also cut a hole out of the top shape and put a picture in the "sandwich" of felt to make a frame.