Who Stole My Seat?: The Invisible Chair
The invisible chair isn't actually invisible, you can still see the metal armature and, more than likely, the polyurethane film that you actually sit on. Still, I would get one and put a potted cactus underneath it for shock value. Then, after my friends (LOL!) have come over enough and gotten familiar with it, I'll replace the film with cling-wrap and watch somebody take an anus-full of spines. Which, if it goes anything like the 'super-glued toilet seat prank', will probably be me (I get drunk and forgetful and only pee sitting down).
Thanks to Tori, who won't sit on anything without testing it with her hand first after throwing herself on a beanbag chair that turned out to be a giant beachball with nails inside.