A big 10-foot-wide, tilted, spinning cylinder filled with multiple densities/colors of sand. You can alter the speed of rotation with that steering-wheel-looking thingie down there in front of the people. Completely mesmerizing to watch! I'll upload video of it in action as well.
September 28, 2012@15:07

IMG_1878
1/30 sec at f / 2.0@6 mm
ISO 500
Canon PowerShot S95
6.0-22.5 mm