Multistage Rocket Trolleys
A pair of trolleys with very low friction, so you can throw fuel one way and the trolley will go in the other - a rocket.
This is a classic demonstration of conservation of momentum, which we built for the christmas lectures in 2016 (48:00).
This can be used to show the tyrrany of the rocket equation. The more fuel you add, the less advantage it ives you.
As an extra twist we built a two stage version which appeared with NASA's commander Hadfield as the engine of the first stage.
Also:
Demonstration
Commission