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.