Our version uses ping pong balls that can be blown up tubes, and then controlled on their way down by channels, that stick to a steel wall.
It has been incredibly successful, and will fascinate 2 year olds, PhD engineers, and everyone in between. Fans in the tubes blow the balls back up to the top which gives an immediate task - to make the balls keep circulating. Doing this once is quite easy, doing it reliably is much more difficult, and involves using trial, improvement, and empirical learning, all vital for real engineering.
This is our latest version