I decoupaged numbers into the bottom of an egg carton. I used pom-pom balls, but small rocks, buttons or whatever would work. Just make sure your kids don't put things in their mouth.
Counting/Number recognition. I decoupaged the paper onto some cardboard, then tied string with a knot on the front and back of the card so it can't be removed. I cut slits on the right side and they have to slide the string into the correct slot.