Get the class to write a love letter that Shrek might write to The Creature From The Black Lagoon's sister, the object being to get as many humorous points across with the straightest face possible, i.e. "When I saw you today, your tooth glinting in the moonlight, my stomach convulsed, my sphincter spasmed and let loose the softest, most silent, most deadly bottom burp, in your direction. I instinctively knew, as your nostrils twitched, that you had smelt it. I would dearly love to see that beautiful, monstrous lip curl again. Do I hope in vain?"
Something like that
