Stereognosis - is the ability to perceive and recognize the form of an object in the absence of visual and auditory information, by using tactile information to provide cues from texture, size, spatial properties, and temperature, etc.
The Mystery Bag (3-6 years) from How to Raise an Amazing Child the Montessori Way;
The mystery bag has long been a favourite children's activity. Usually it is simply a cloth bag or box with a hole for your child's hands, through which she can touch and manipulate objects that she cannot see. To play you will need a collection of small object with which your child is familiar and which she can name. While she closes her eyes, place an object inside the bag and challenge her to identify it by touch alone. If your child guesses correctly, you and your child switch roles. Keep this game going for older children by making it more difficult, using different coins, shells, or geometric shapes, for example.
I started using the Mystery Bag with Otis as a toddler. Now he still loves it but to make it more difficult we increase the number of objects and also use it as a language tool (this is great with items like the geometric solids).
For us playing this game is more than concentration, a toddler working - it is about Otis and him isolating his sense of touch. An activity in which I can see his mind at work as he feels the items in the bag and the pleasure he receives as after he runs his fingers over the cool stone or brings the feather to his face.
We know as the child touches the object they form a picture of it in their mind - this picture builds as the information increases until they get to the point where they can identify the object, without the sense of sight.
Basic Montessori - Learning Activities for Under-Fives contains extension ideas including the sorting of items from three bags blindfolded. I haven't tried this with Otis - if I do I will be sure to let you know how it goes.
At home we use the Silk Lined Language Object Bag and the pictured Montessori Stereognostic/Mystery Bag. Both of these bags have a silk-type lining which can highten the sense of touch. However if you can find a simple drawstring bag this activity can be done with items from around the home.