A variant on Opposite Day was the Opposite Game. The initiator of the game would ask, ofr example, 'Do you shag your dog? By the way, we're playing the opposite game.'
Answer No : means you shag your dog, because we're playing the Opposites Game, so no means yes.
Answer Yes : means you shag you dog, because we're playing the opposites game, which means that to say "we're playing the opposites game" means "we are no longer playing the opposites game". In which case yes means yes. So you do shag your dog. And you don't even have the decency to try and hide it.