Math in Art Festival I did with Susan Walborn (an amazing teacher who's moved on to becoming an amazing retailer - must just be amazing, eh?), one of my favorite lessons was a mobile lesson (link leads to a pdf of a verrry complete 3rd grade lesson plan) based on the art of Alexander Calder and the math of area and average.
The emphasis is on the average as a balance. In calculus today, we covered center of mass, and built the connections among the moment, the weighted average and the idea of balance. Students used the ideas to create cardboard cutouts of curves and find the balancing point. They did a great job. Mine is the unimpressive cubic.