For some reason one of my most memorable cereal box toys was a baking powder submarine. Pop open a little chamber and sprinkle baking powder inside. Close it up and drop the sub into a 2-litre bottle filled with water. The sub would sink to the bottom but the water and baking powder would mix and form a bubble. That would take the sub back to the surface of the water where the bubble would pop and send the sub down to the bottom again. It was such a simple toy but pure magic to a little kid.