My sister has a farm filled with old-growth oak trees and tons of scrap lumber. It just had to be done. I may be one of the few people in their 30's who has their own tree fort.
It has a fold-down Ouija board table, two large benches which can accommodate six drunk adults (we tested this), and a large rope swing. And writing on the walls is heavily encouraged in "Fort Awesome."
Fort Awesome has a BB gun shooting gallery on its underside: zombie, robot, monkey, and squirrel aluminum targets that make a satisfying "ping" sound when pummeled with a BB gun. [click on photos to embiggen]