Plaster of Paris, that is:
We used some beach pail/sand castle molds I picked up at the dollar store, and used them as molds to make castles. My oh so fancy mold-release? Cooking spray that we also picked up at the dollar store.
We'll wait 24 hours for them to completely dry, and then paint and decorate them. (I'm leaning toward a haunted castle look for mine. It's only like two and a half months 'til Halloween, y'know!) We added a squirt of blue tempera paint to the last of the plaster, and put them in little foil stars to make some sidewalk chalk:
And here's some sidewalk chalk work from Bannanarama, whose current obsession with anime has led her to begin learning some of the Japanese language: