Zorb Ball (or Zorbing) is the activity of rolling down a hill inside a giant inflatable ball. It looks like fun, see this video of some zorbing in New Zealand, where the sport seems to have originated.
I am not sure if this one makes the classification of an actual sport, though much of the publicity about this activity describe it as an extreme sport.
It is quite extreme, but there is no competition about it, unless you compete with your friend about whether you will do it or not.
It must be a safe activity, you are totally protected by a big balloon.