Sonic Adventure BopBags. If a bop bag is not random, I'm not sure what is. This is an inflatable weighted (at bottom) punching bag that bounces back and knocks over your items.

Why do you need a bop bag? Ever? But, here they are. 48 inches high and guarenteed for seconds of entertainment! These are by the same people who did the pens. They were available at Big Lots and probably elsewhere. I was never able to aquire one.

A Knuckles punching bopper I could understand, I mean, the whole fists thing...but Sonic? And why not Eggman-- punching him out would be relaxing. Maybe there were more of these, like one for each character. They weren't too popular, or common. You can occaisionally find one on ebay, though.