I think you could do an audio-looping app. You'd make a loop up from a series of instruments, but the instruments would appear consecutively. You choose the loop type (3 bars of 4-4 time, say, I'm not a musician!). Then you get an announcement "clap", with a metronome sound in the background, you tap the phone for a beat. Next the app announces "snare" and you tap for each snare, etc.

Maybe it works best by giving you a few bars of a tune and you make the percussion up to fit?

Whatever, I think there's something there.