Since dry ice is frozen CO2, it can be used to carbonate water. Place a small amount of dry ice in a container and fill it with room-temperature water. As the dry ice submlimates into gaseous carbon dioxide, the water will begin to bubble and the temperature of the water will drop. Ice made from water may form around the dry ice: use a spoon to break up the water ice. When the dry ice is gone, taste the water. It should be mildly carbonated. While the ice is submlimating, do not seal the container, as gaseous pressure will build inside it.
Fill a glass or other suitable container with water and add a squirt of dish soap. Use tongs and wear gloves to place a few small pieces of dry ice in the water. As the dry ice sublimates into gaseous CO2, bubbles will surface and burst into puffs of smoke. The soapy water has more surface tension than plain water and so will form larger bubbles that will rise above the surface. For added dramatic effect, you might add food coloring to the water. A glow stick will add more of a Halloween aura to the experiment.
Take a balloon and inflate it using your own breath. Have a friend or assistant hold a bottle upright. Then, wearing insulated gloves and using tongs, drop a few pieces of dry ice into the bottle. Take another balloon and cover the opening. When the dry ice sublimates from solid to gas, the balloon will inflate from the dry ice alone. Once the balloon is inflated, remove it from the bottle and tie it off. Then, toss the two inflated balloons and compare their motion. Since carbon dioxide is heavier than air, the balloon inflated with dry ice will fall much faster than the one inflated with your breath.
Wearing insulated gloves and safety goggles and using tongs, take a plastic 35mm film canister and place a few pieces of dry ice inside. Reseal the canister and back away. Make sure any class members are at a distance from the canister and that they are wearing safety goggles. Wait a few moments as the ice sublimates into gas and pressure builds inside the canister. Soon, it will pop its top and may fly as far as several yards.