Rock Climbing. It's like hiking, except vertical.
This fun sport can be picked up at any age. I've seen 4 year olds scale walls like they were spiders.
One of the most reknown climbers was Dan Osman. He was famous for his daring free-solo climbs around the world and his "Flossing the Sky" stunt where he would jump from a cliff and fall hundreds of feet allowing his climbing ropes to catch him, similar to bungee jumping. Dan Osman fell to his death doing this. It was innevitable really, I mean come on. He was jumping from god damn cliffs with just a rope.
(I'm linking this to other topics that aren't really related because there aren't any good related topics yet!)