1 player. Using the rhythms of any familiar tune that you can play by heart (all right, use sheet music if you absolutely have to) invent a new melody. Your choice of tonal or atonal. You may change the tempo to slower or faster, but keep the relationship of the note values. Advanced players may change meters. Repeat in all keys, creating an entirely new melody each time. Vary the style if you can (e.g. if the original was a folk song, make the new one a calypso, march, waltz, or dirge, etc.).