I waited a little too late in the month to go and find plastic ornaments though, because I couldn't find the plain, round plastic bulbs anywhere, I seriously looked at three or four stores before I settled on some that had a design painted on the front that I found at Target. I also put out some trays with different foam shapes, craft pom poms, and little Styrofoam beads (messy!!) for the kids to sort through and fill their ornaments with.
Some of the kids had a great time filling and then refilling their ornaments, some filled theirs to the brim, and some were happy with just a few items inside of theirs. It was fun to see what all the different kids did!
And here is my son's little masterpiece: