Hot air balloon
This is an old demonstration that I first saw whilst at school. Back then the teacher used a bunsen to heat the air inside the bag (and nearly set fire to the lab in the process!) - a friend suggested the idea of using a toaster instead, which appealed to me. The only issue I have found is that the air does not get hot enough to list many types of bin bag - the cheaper (economy) brands tend to work best, as they are much thinnerand hence lighter. The demonstration is an excellent way to round off (or start) a lesson on convection!