Once I found the right sized box this activity took me less than five minutes to make. A cardboard box with two holes (for the chid's arms) = Montessori Mystery Box. This is a fun game that depending on the child can last anywhere from five to fifty minutes.
The idea is to isolate the child's sense of touch. They can identify objects by touch only. We like to play this in lots of different ways. By the child feeling a box with lots of items. As they touch or discover with their hands each object they try to identify it. Or you can say to them "can you find me the pinecone" and they need to search for that one item.
Or the child can have one object and identify it by touch. We like to use household objects, fruit and vegetables, model animals, items from our nature basket (a soft feather is a nice contrast to a rough seedpod, a cool metal spoon is a contrast to a soft play silk). As you can tell our geometric solids are also popular right now. If the child tries to peek in the arm holes paper or fabric can be attached to the edge of the cardboard cut-out or you could use felt or paper like at this DIY post at Family FECS or using a tissue box as at Montessori Moments.
Otis says "this is a cone", I ask "why is it a cone", Otis replies "it has a circle here and flat here and a point here" (as he touches the wooden shape). It's a lovely, peaceful activity to do together.
Why would we bother playing a game like this? Maria Montessori in The Discovery of the Child writes at length about the importance of educating the senses "The training of the senses must begin in the formative period of life if we wish to perfect them later through education and make use of them in any particular human skill".
"To learn a craft, especially if it is of an artistic or refined nature, means that one must develop his senses and the movements of his hands, and these movements are assisted by a consequent refinement of the sense of touch". She further goes on say the ages of three to six is "when there is a natural inclination to perfect one's senses and movements".