Sound Drop. It's really quite incredible and I'm surprised that it continues to be free.
The app combines music with geometry, physics, friction, gravity and more. Balls drop and create sound as they strike against the lines you create. Sound varies depending on angle and velocity. On the one hand, it's quite simple; all you have to do is draw lines. But with practice you can turn the app into a sophisticated, modern instrument.
This app is incredibly unique. Go grab the freebie and try it for yourself. It's fun!
P.S. If you're doing Workboxes, this would be a fabulous 10-15 minutes activity.