All my posts on Archimedes' Principle, water displacement, density and buoyancy are here...
Archimedes' Bath - This covers the maths and also how to make your own Archimedes' bath out of an old ice-cream tub.
and beans. Teaches about density and buoyancy.
How to make a fresh egg float
The Buoyancy Test!
What is density
How to make a simple Hydrometer
Layers of liquid - Another fun experiment about density
Liquids changing places! Works like magic!
Make a Lava Lamp - this is another fun way of explaining about density and buoyancy!