A common, but educational project would be making a battery out of a potato. Other projects include making variously shaped paper airplanes and seeing which fly better and why.
Some include bubbles (experiments in water tension), or the soap-powered boat (again, water tension changed by soap). Or you could do something with density (water and oil). Simple and easy!
A cool and esy one that is very visual is to show the magnetic lines of a bar and a horseshoe magnet. You put the magents under a peice of white paper and then sprinke iron filling on top and then you see the magnetic lines.
