I made "dino treasure eggs" for the kids to have on their ProD day here at the centre. 1 cup dried used coffee grounds, 1 cup flour, 1/2 cup salt, and 1/4 cup sand (don't quote me on this, though), mix with water, form into ball, stick prize inside and roll again, bake for 3 hours at 150. I thought it would take them a good 20 minutes of carefully excavating, scraping, and digging to get the prize (fancy rings 'cause I couldn't find any dinos small enough to fit!) - took them 2 minutes, tops! But they had fun!!