I would like to try http://www.solargraphy.com as a project with my elementary school-aged kids. Sounds like a lot of fun -- build a bunch of one-off pinhole cameras, distribute throughout random places, and expose for weeks or months.

However, the technique relies on scanning the exposed photographic paper to get an image. Is there a way to get super-slow (month long) exposures with a pinhole camera using wet chemistry and a darkroom? I assume that paper negatives (or positives?) would be part of the process. I'd love to show the kids the magic of the image appearing in the trays. I supposed one could make some home-spun ND filtering material, but bracketing exposures could take a long, long time, and I'm not willing to sacrifice a real ND filter to a camera that's left outside for a long time.