A round hovercraft toy like this one here is a fun way to teach about friction and forces, angular momentum and other ideas open to your imagination.
Paint the hovercraft a solid color and mark an X at the center and an X at the edge and observe how fast each X rotates.
Compare how easy it is to get the hovercraft moving by pushing it in a straight line when it's floating on a layer of air, compared to when it is sitting on a table.
Do you have any recommended science experiments using inexpensive materials? Did you find these ones useful? Let us know what you think.