[From Paul R. Nevergold]

Boiling water in a paper tray is fun.   Set up a bunsen burner and a ring stand with a wire screen.  (I used to use a camp stove because my room did not have gas.)  With out talking, fold an index card into a tray taping the corners.  Fill the tray with water and slide it onto the screen which should be glowing red and wait for the water to boil.
Among other things, it illustrates the heat capacity of water.

I also made a 5' density tank out of 3/8" plexiglas.   You can show beautiful density currents which can be used to illustrate cold fronts and upwelling and other things as well.

If you are handy or friendly with the shop teacher you can build your own.  If interested, contact me off the listserve and I will send you the dimensions.

Note: I was a geological technician for 25+ years but it wasn't until I taught Earth Science that I realized that the whole Earth - solid, liquid and gas - works on density differences.