For work on volcanoes: Burn off the plastic and paint from a beer bottle top, place it in a fume cupboard, pile up a small ammount of ammonium dichromate and light the top with a bunsen burner (be careful not to set you hand alight, like I did the first time I did this!!). Watch that volcano go! It is fantastic, but because of the ammonium dichromate can only be demonstrated to the class (and you need to be careful with clearing up as well).

One was described to me about shaking cola cubes in a big tube to look at rocks weathering, and conservation of mass.