Gibbon Slacklines
Slacklining: it's not just for climbers anymore! It's also for hippies. But it's not just for hippies anymore either! All you need to take part in this exploding sport/art/activity/whatever is two trees, one slackline, and a rudimentary sense of balance. Judging by the guys we see doing it in the park, it helps to take off your shirt, too.
Gibbon Slacklines official site