I recall reading a book many years ago (Airport International, I think) that had the following trick.
Flight deck requested a tin of vegetable soup (this probably ages the story somewhat!). During cruise the new hostie was called to bring a sick bag into the flight deck as one of the crew was feeling unwell. Tin of soup was emptied into the bag and the unfortunate newbie was called to collect the bag. As it was passed back to her the captain would dip his finger in and taste it. "Mmmmm...tastes good"
My favorite story came from a captain that I trained with. During the cruise they rolled a twenty pound note into the ACARS printer. When one of the crew came up, the captain started loudly complaining that he had been underpaid by £20 on the last nightstop and that P2 should get in touch with the company to rectify this. On the cabin crews next visit, the F/O would press the ACARS feed button and, hey presto, £20 appeared. Not sure if anyone ever fell for it, but a good way to pass the time when crossing the pond!