Montessori Mama suggested that we try a mortar and pestle. Super big, ahem, MAJOR hit in our home. After showing him how to use the pestle to gently crush egg shells, he took it out of my hand. He commenced to bang so loudly that Hubby came out of his cave, um, office to see what was the matter.
Once the egg shells were pulverized he said, "Momma, let's do Cheerios!" Following the Cheerios he wanted to do nuts. When everything was amply demolished he said, "I need some milk to eat this." So, we poured milk.
Then he said, "You know what's missing? I need to smash some cranberries to add to this." Dried cranberries don't change their shape too much but it was a fun experiment.