Place some yeast, sugar, and warm water in a flask (or bottle with small neck). Quickly place a balloon over the flask opening and allow it to sit for the class period (or longer).
At the end of the period, you will find the balloon has inflated. It is filled with carbon dioxide released during cellular respiration.
You can prove that it’s carbon dioxide and not oxygen: light a wood splint or popsicle stick on fire and then blow it out so that it’s just glowing. Release the contents of the balloon onto the glowing splint. If it’s oxygen, the splint will return to burning; if it’s carbon dioxide, it will go out.
A note about my pictures... In the picture of the initial set-up, the bottle only has a little water in it. It wasn't enough - the yeast respirated, but the carbon dioxide they emitted took up the remaining space in the bottle and didn't make it to the balloon. I redid the experiment, filling the bottle much fuller, which resulted in an inflated balloon; but I didn't retake the set-up picture.