Write a piece for three instruments you wouldn’t normally think to combine (for example, English horn, tuba, crotales).
The twist: each “part” is actually a fully formed piece in itself.
Have them begin together. They will end whenever the last remaining voice finishes its piece.
All combinations, then, are left to chance. Did you hear anything that really engaged your interest? Would you leave your “music of simultinaiety” as it is or as a springboard for more conscious development?