1. A friend in Canada was in a Cherokee Six with a mate and they were doing zero degree parabolas. Taking it in turn as P1, with the other going in the rear and photographing themselves floating in the middle of the cabin, then swapping pilots!
Never tried it before myself, but am very tempted!
2. Handing over control to inexperienced people on long flight legs and doing the straight / level exercise with them, getting them to head for a point in the distance. Gradually applying surreptitious rudder to put them off course or to make the target move from side!
3. In a continuation of 2, teaching people to fly constant level turns and putting in bootfuls of rudder to confuse them!