Cartesian diver
The Cartesian diver is a classic demo, which involves both pressure and buoyancy.
It is a very neat demo, and the carbonated drinks bottle makes it easy to build and robust. But to understand the demo you need to be able to connect squeezing the bottle to increasing the pressure in the bottle, to squashing the bubble in the diver, to reducing it's buoyancy and making it sink. This is a lot of steps to understanding. So we have added a simple pressure gauge at the bottom of the bottle making it more obvious what is going on.
Tabletop exhibit
A cartesian diver as a simple exhibit to go on your tabletop
£300
to £500