Magnetic Ball run
A ball or marble run is a good exhibit, but if you add a way of lifting the balls back up to the top again you get something great.
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
Or some much older versions
Ball run testing in 2013
The ball run in Cambridge Science Centre