You really need to define wht you're looking for in "tough". As someone pointed out, boxing is pretty tough. Would anyone here race their bike against a pro cyclist? Even though we'd lose, I'd bet many would. How many of you would put on boxing gloves and step in the ring with a pro boxer? Your cycling days could very well be over in a few minutes. It all depends on what is being asked by "tough".
Personally, although it doesn't take any physical fitness, I think cordless bungee jumping is about as tough as it can get.