My son has gone through a few bouncy ball kits and it is still one of his favorite things to do.
This is the kit he got for his birthday and him and daddy made a few different bouncy balls.
This is a Rose Art kit and it is pretty easy to do. All you do is fill up the ball molds with the crystals, soak in cold water, then let set and you have bouncy balls.
Be sure to store them in an airtight container to keep their bounce.