There are many conditions and people who find it easier to concentrate or be calm if they have something to fiddle with. There are also those with physical issues with their hands or in rehab who will find manipulating things and therefore stretching and moving their hands a positive action. I have known people fiddle with pens, stress balls, muscle balls and massage balls and all of these are valid but I would also like to suggest Tangles.
Tangles are traditionally a toy given to babies when they are teething as there are no sharp edges and no way for them to hurt themselves on them, but there are several versions made for those with bigger hands. A tangle is at a series of curved 90 degree turns which can be set at any angle to each other. This means they can form a variety of shapes and be sized from very small to covering quite a large area.
Traditionally they are made of smooth plastic but versions now exist with different textures, from rough plastic to fur and many in between. Some tangles can be split into individual sections giving another challenge to the fingers and some are designed not to be disassemble. They are available on amazon in many different types such as Therapy Tangle, Tangle Hairy and the slightly larger Tangle Creations.