Balloon Popping meter: as the children sing well, a leader pulls on the ribbon from behind. If the kids have mastered a song, the balloon makes it to the top, where it is popped. If all the songs are mastered, the kids get tootsie "pops.
Games & Icebreakers - Our youth group did this as a competition--who could create the best tree. We added real lights too. They loved it! Lots of laughs, and no real need to buy supplies, other than crepe paper.