2 players. Players pick a major scale. Player One plays quarter notes, stepwise only (no leaps), deciding in the moment when to turn around (no repeating of notes). Player Two plays in the same key, but is completely free in note and note value choice.
Idea 1: Player One plays only in the low register – like a walking bass.
Idea 2: Player Two syncopates as much as possible.
Idea 3: Player Two plays mostly long tones.