Disclaimer: I'm a bit reticent about posting this as a separate idea, because I am so deeply indebted to Dog Ed's Collaborative Composition Jukebox for inspiration, modified by Mephista's suggestion that "your body should be involved." I think maybe it's just different enough to stand on its own, but
if there's popular dissent, I'll repost it as an annotation to Dog Ed's idea.
Like Dog Ed's device, it's a music machine with an a.i. composing algorithm, stationed in a public place like a park.
Here's where it differs:
The machine has a sophisticated motion sensor, and automatically begins playing whenever someone passes by. It composes music that fits in with the person's motion, synching the beat and the "mood" to the person's pace, whether walking, jogging, hopping, bicycling, somersaulting, etc.
If the passer-by is aware enough to notice what's happening, he stops. The music stops, too, hanging on a suspended chord, awaiting the person's next move. After some experimentation, the "user" is now dancing away, but the more he puts into his dance, the more "dancey" the music gets.
A second person happens by. The music machine senses her, and alters the music to attract her into the dance, too.
Soon many people are all dancing together, and the music they're dancing to is being created by the combined inner joy of them all, as expressed by their dancing.
Man, I'd love to see something like that.