The recipe is 1 cup coffee grounds, 1 cup flour, 1/2 cup salt, 1/4 sand, and 3/4 water. I doubled that for this project. We don't drink alot of coffee in my house and when we do it's egg coffee, so I had to save grounds for about 2 months, in the refrigerator, of course.
We wrapped the treasures in plastic. The boys got Hot Wheels and the girls got rings.
Then we molded the clay around the treasure and bake them for about 30 at 170 degrees. I scared to go any longer; I thought the toys would melt. The object is to get them go and hard so the children have to chisel their way in. I plan to put them in a plastic treat bag with a cute tie for giving.