Scottly, your a friend and I would never prank you bikes,
the sardine one is kinda nail in the coffin for a friendship as one that heavy putrid oil burns into the exhaust it is gonna take some harsh measures to eliminate it.
Visual side benefit, all manner of cats will hang around the bike, climbing all over it.
Another simple one would be to pull several vale inspection ports and leave them on the seiat. He'd have freshly adjusted valves either before he left or shortly afterwards. The ride home would be pucker listening for abnormal sounds.