Depends what you want from an extreme sport. Lots of quick, controlled thrill stuff out there. For real slow, complicated mind games, and the certainty that nobody is coming to get you if it goes pear-shaped, I recommend cave diving. A couple of hours deep into a mountainside underwater knowing that your nearest fresh air is the same couple of hours away gets the hairs on the back of your neck going. Alternatively try home-brewed mixed gas deep diving on some nice wreck. In terms of thrill for your buck, you can't beat that sort of stuff. Quick thrills, throw yourself out of or off something!! Alternatively, cheat on your wife .....