Use a toaster, cardboard and big bags to demonstrate hot air (which is lighter than cold air) rises
Toaster/ Protective cardboard/ Bin bags
Cut and fit a protective collar of cardboard to the top of the toaster. With the toaster turned on and the bag placed carefully over the cardboard, the bag will fill with hot air and slowly start rising. It will no go far before the top collapses and it falls back down. This can be repeated with another bag with sellotape round the base of the bag to make it slightly heavier - this will stabalise the bag and it will rise much further much to the delight of the group.
Warmer air molcules have more energy than cold air and this make the air and the bag rise.