3-4 players play long tones, changing at will.
Variation 1: Pick a key to limit the choices of notes (e.g. C major scale).
Variation 2: Pick keys independently.
Variation 3: Leave the choice of notes completely free.
Variation 4: Make the notes as short as possible and widely separated.
Variation 5: Alternate widely spaced short notes with continuous long tones.
Variation 6: Instead of any note in a scale, restrict note choice to chord tones only (1 3 5), but make choice of key open.
Variation 7: If your instrument is capable of it, change timbre either with every note or now and then.
Variation 8: Instead of smooth voice leading, try wide displacement of consecutive notes.
Variation 9: Add some kind of percussion to the long tones, creating a steady pulse accompaniment.
Variation 10: Pick one other player to listen to. When they go up, your next note goes down. Or vice-versa. Change player choice now and then.
Variation 11: Add dynamic phrasing to a series of your long tones: crescendo, decrescendo, or hairpin.
Variation 12: Pick one other player to listen to. Play the same dynamics as they do. Or: play the opposite. Change player choice now and then. One more idea: combine this with Variation 10 (note direction) and either do the same with both (i.e. play the same direction and same dynamics) or the opposite (same direction and opposite dynamics or vice-versa).
This game will sound better if players are occasionally silent (in fact, silence improves almost any group game. The more players, the more each should be silent.