Not an FA joke, but one that I heard from an old C5 aircraft commander- The crew had spent several days overseas in the far east and apparently consumed a large amount of beer on the layover. They saved all of the empty cans, and stowed them away up under the engineers panel in large garbage bags.

Headed home, with a full load of troops and other gear in the back, they had the loadmaster walk by the troops every 10-15 minutes with a tray full of full beers (unopened) on his way to the flight deck. He'd then swap out the full cans for the empties, and then walk back past the troops. Apparently made for some good conversation in the back...

Another good prank is to make the FA salute the US flag painted on the tail of the aircraft every time they board. Had a former Navy Commander do that one.