One of the things I love about flowing water is that it is so scalable - a little stream flowing through sand acts the exact same way a huge river does in the same circumstances. I just happened across some really neat looking stream tables at this website. These could be a great outreach tool. Granted, as a graduate student I can't afford to buy one of these, but maybe some day I can get one or make one on my own.
Doesn't this look fun?