To demonstrate the expansion of gases, find two jars, and a bottle that can fit into the jars. Fill one of the jars with cold water and one of them with hot water. Place a balloon over the end of the bottle so that the balloon hangs limp. Now put the bottle into the jar of hot water. If there is sufficient heat and gas inside the bottle, you should see the balloon being inflated, due to the expansion of the gas inside the bottle. Now move the bottle to the jar of cold water. The balloon should deflate.
To demonstrate how hot air rises, build a candle-powered hot air balloon. The balloon itself can be made out of paper or plastic. Ideally it should be stiff to minimize the likelihood of burning the balloon when placing the candle underneath it. Leave an opening at the bottom, and give the entrance a circular frame made of wire. Suspend a candle in the centre of this circle; more wire can be used for this. Light the candle and allow it to fill with hot gases. These should cause the entire balloon to rise, if the balloon itself is not too heavy.
If you are in a hot room on a cold day, you can demonstrate the currents caused by hot gases by lighting a candle and holding it to a window. When you open the window, some of the hot air within the classroom will escape through the window, causing the flame of the candle to point towards the open window.
This last one is just a bit of fun to engage the people you are teaching. Ask them to simulate Brownian motion by going to an open space and slowly moving around, rebounding off each other in random directions when they come into contact with each other. Gradually increase the "temperature" of the "room" so that they move faster and faster. Be careful that no-one is reckless.