Something to do with kids and a pinhole camera and the sun...

Let the kids make a regular paper negative pinhole image, maybe using a simple tin cylinder camera. Have them make the exposure with the camera pointing away from the sun. With a paper negative in a 3 - 4 inch diameter tin, this would take 15 - 20 seconds depending on the paper (use Foma Variant 312, it is top notch for this purpose). They can develop the photo just like normal. Then have the kids make the same image, but for the last second or two, point the camera at the sun and move it slightly, maybe waving the camera back and forth or in a circle. The sun will quickly burn a trail on the negative that will be like using a magic marker on the scene. They might like it. This experiment would show the power of the sun's rays, that's for sure. (I've done this a few times as a mistake; but as a lesson? Might work great.)