Slacklining is a bit like walking a tightrope only lower to the ground (and the line is slack not tight).

 It is reputed to help core balance, mental concentration and smoothness of motion. I think it would improve strength and agility too.

 It isn't easy to learn so you may need a bit of 'stick-at-it-ness!' but it is quite addictive! We found this was a fun sport for the whole family and one site claims to have taught people between the ages of 3 and 84!!