The picture above taken at night is a long exposure shot of a tennis ball with 3 sparklers coming out of it being shot into a lake. Our original idea was to shoot a flaming tennis ball, but we settled with sparklers. We also tried glow sticks wrapped around the ball and trailing the ball like a tail, but the glow sticks were not bright enough and could not be seen through the camera.
This sling shot will shoot a tennis ball about 125 yards and golf ball closer to 200-225 yards. Unfortunately though, the golf balls cannot be retrieved in the lake (because they sink) so we rarely shoot them. Everything else that we shoot is retrieved. Usually a few of us go out onto the lake in kayaks, and we try to catch the tennis balls as they are launched.
Extra-Mini-Instructable: In the daytime picture above, you can see a fitness trampoline at the end of the dock. We built a frame that sits on the lake bed and supports the trampoline so it sits level with the dock surface. The frame isn't very rigid because it is over 6 feet tall and built from extra pipes and hardware, but it is still really fun to launch yourself into the lake with the help of a trampoline.