Things I learned:
No accidentals (must be in the key of C and free of any sharps or flats).
You can't play a note and the note directly above it because the hole punches are too close.
You can't play the same note in rapid succession (on adjacent lines on the grid's 'time' axis).
Now I really want a music box that can play half steps, can play notes in more rapid succession, and isn't so squeaky.