slack lining

outdoor rock climbing / bouldering (all you need is a crash pad)

Look into joining a town's conservation group. Most do regular hiking/biking trail maintenance and need all the help they can get. It can be fun if you're into that type of stuff.

Metal Detecting for artifacts (just pickup a $150-400 metal detector)

Gold panning

Build an Axe Throwing target out of stumps and have fun axe throwing or learning how to throw knives.

Photography