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.
I love "practical" applications of a skill. Church musicians are real people, not some hypothetical conversation piece, and I bet that somewhere, someone needs a cathedral music librarian. It just sounds like a fun job!