Once the infant is crawling and sitting, everything changes including the toys and materials we present to them. With materials like the pincer and palmer blocks, the egg and cup, the fact that the child is sitting independently to work, is more important to me than their age. Here are a few materials that Otto is using now that he is sitting.
Above is the Wooden Mini Sorting Box. It's not specifically Montessori but it's great for the child's need to post items, to open and close and for coordination. I've only seen this in the UK but it's a reminder that you can find lovely materials outside Montessori specific shops.
Object permanence box (ours is c/o Montessori Shop). This has been a favorite for weeks now, I don't know a child who doesn't love it. You put the ball in the hole and it rolls out into the tray. It's obviously all about object permanence but will also build the child's concentration and coordination, this action is often repeated many times.
Imbucare Box with Ball (also c/o Montessori Shop). This is a little harder as the child needs to retrieve the ball from the drawer.
Otto has a need at this stage to bang things together, often bang objects together like two blocks. Our drum is well used too and I'm sure will be on our shelves for many months, perhaps years, to come.
Once the child starts crawling it's fun to make little obstacle courses. Have the child crawl over pillows or through tunnels, you can see Otis using his much-loved tunnel here. We've made a tunnel using play silks over our climbing arch (c/o Wiwiurka).
Once the child is sitting they can explore so much more with their hands. Baskets such discovery baskets are also important, I'll be sure to share some of Otto's baskets soon!