What's more summer-y than climbing a tree? As kids, we used to scale the old willow out by the railroad tracks in our back yard. We'd swing on the vinelike branches, but we couldn't get very far out over the hill. The neighbors had a rope tied to a branch in one of their trees and there we could try to swing out and bounce off the train with out feet as it passed. I was a bit of a chicken, but there were kids in the group who accomplished loud thuds. Luckily nobody died.