Sprinkler. Either put the hose on the tramp or create this cool sprinkler. Source: Instructables
Crack the Egg. Have one person sit in the middle holding themselves in a tight roll up position. All others try to crack them as they try to remain in one tight ball.
Super Heroes. Grab some capes or sheets and let the little ones run around. they can flip and fly without crashing and getting hurt.
Hop Scotch. Using chalk, create a hop scotch and grab some friends to play with.