An improvisation exercise: Easiest with keyboard Midi controllers. Two or more people play keyboards together, but with no sound output. The Midi is recorded for playback. Keep practicing until it sounds good.
You could also do this with acoustic instruments, or an entire band, if you had really good hearing protection to block the sounds for the players but not the recorder.
Variation: Using closed cell headphones, all the musicians would be hearing the same soundtrack, for example, “Free Bird”, but unable to hear the instrument they are playing