1c used coffee grounds
Mix it all together (dry first, then add the water) until it forms a dough. Break off pieces and form a ball, press a 'treasure' into the middle and seal with some extra dough. You can either bake them for about 15-20 mins at 150 degrees or air dry for 2-3 days until hard.
Arm your kids with some little 'tools' and let them chip away until they find their treasure. We'll be trying ours out in the next few days.