Hidden Treasure Rocks!! To make them, take 1 c leftover coffee grounds, 1 c flour, 1/2 c salt and enough water to make a dough. Hide little trinkets inside and bake at 150 for 15-17 minutes. Or, set them out in the sun all day. Cool, huh? Have a rock hunt. If I were a guy, I would hide a diamond in one. How cool would that be?! A rock in a rock. So clever.