Carbon Dioxide Source
A water seltzer or bicycle pump with a carbon dioxide cylinder inside is a great source of pressurized carbon dioxide that can be used to demonstrate countless different ideas.
One dramatic experiment you can do to demonstrate the higher density of carbon dioxide gas compared to air, is to fill a large tank, such as a fish tank, with carbon dioxide and blow some soap bubbles on it. The soap bubbles will hover on top of the invisible carbon dioxide layer.
You can also experiment with carbon dioxide and water, adding an indicator to see how the pH changes when carbon dioxide is bubbled through. Bromothymol blue is great to use as it turns a bright yellow. Or, attach a hose or aquarium tubing and pump the carbon dioxide through lime water to produce a precipitate. These experiments can also be done in a fish tank to make for an exciting demonstration for the class.
The carbon dioxide pump can also be used for inflating objects such as basketballs, for physics experiments.