We also have salt and flour, each each with a koolaid packet mixed in for scent and color. Other great ideas are brown sugar, oatmeal, cornmeal, cocoa, peanuts (watch for allergies), bacon bits (very scentful) and we combine water with a flavored extract in paint cups with droppers!
Notice the mortars are on trays. This is done to give the children their own space and keeps the mess to a minimum!! When a child is done with this activity we either empty their mortar into the trash or back into a bowl for another friend to use.