Normally you can’t see carbon dioxide because it is an invisible gas. But if you get some frozen carbon dioxide (also called dry ice), you can see it as chunks of ice. You can get dry ice from some grocery stores, or ice cream stores. Put it in a styrofoam cooler, and keep it dry.
To see the carbon dioxide ice turn into a gas again, you could just watch it melt without making any water. To make it more exciting, push some small pieces of dry ice into a balloon, tie the balloon, and throw it into a bathtub or a swimming pool full of water. At first the balloon will sink, because of the dry ice in it, but as the dry ice melts the balloon will become full of carbon dioxide, and it will rise to the surface. When you pop the balloon, the carbon dioxide will go into the air.
You can also drop a piece of dry ice into a glass of juice or water to make it fizzy – that’s what makes Coke fizzy, too. When your Coke goes flat, that’s because the bubbles of carbon dioxide have popped and floated back up into the air.