Carry-the-stones

Take the kid outside. Optionally hand them a basket or bucket.

Pick up a small stone. Carry it ten feet. Put it down. Walk back.

A 2-year-old will move every loose object in a five-yard radius from one pile to another. Then back. Then to a third pile. A 4-year-old will categorize — big rocks here, small ones there. This is the schema in its pure form.

Make two clear "piles" with a tape line or a chalk circle. Add a rule ("only round ones in this pile"). Move sticks instead, or leaves.