A snowstorm, with the flakes swirling, and the snow piling up at the bottom of the display. A lava lamp. [Received an e-mail later: someone has done this one.] A wave bar, or whatever those things are called that you see in science stores - the rectangular bars that tilt up and down on either side of a fulcrum, with imiscible fluids of different colors that make waves. A cigarette, with smoke curling up and off the display.