The Glass Eye
I wear a prosthetic eye and a good prank could be having Steve-O Swallow it and attempt to throw it back up. It is similar to the goldfish prank only it would be horrible if he couldn't throw it back up. It would have to come out the other end.
Another prank you could try is having Steve-O eat something like cottage cheese with a spoon out of my empty eye socket. There are so many different ideas to try and I am up for anything.