U.S. Patent No. 5,078,642 (.pdf)
Ok, fine, I guess I'm a kid at heart. Just admit it, though: this looks pretty awesome no matter how old you are (artwork aside). It's essentially just several sections of plastic channels that can be stuck to your bathroom walls with suction cups. Add a little water, toss in a bar of soap (bonus points if the soap is carved to look like a stunt car!) and suddenly you've got hours of fun no matter how old you are. I want a setup with a couple loops. And maybe a ring of fire.
Oh yeah, if you click on the picture you can check out the sheer excitement on the kid's face in detail.