Set up a trampoline off of your stage with a bunch of gym mats or old mattresses around it for safety and then set up a basketball ring 2+ meters away from the trampoline. Run a comp where teens bounce off of the trampoline to do a slam dunk. The best dunk will take home the win. Use your crowd to judge who they think is the best dunker.
Buy some cheap foam camping/hiking mats from a department store and gaffa tape them around the poles on the trampoline and also around the vertical pole on the basketball ring. The safer you make this kind of stuff the less chance there is of a teen getting hurt.