I bought a set of Montessori geometric shapes and corresponding shape cards on ebay. You can be a lot less fancy, if you simply own a good set of wooden blocks. I really enjoy how the child can learn “live” geometry, by touching and “feeling” these shapes.
There are a few games you can try. You can download the free geometric shapes cards at freemontessori.com, learn the names of the shapes and match the actual shapes with the cards.
My favorite game is called “An Architect For a Day.” Using the wooden cards, we arrange the “building” we’d like to build later. Then, looking at our “design,” we build, using the actual shapes.