When talking a walk, i usually carry these. It makes me looks like a Asian sage.
don't buy those with decorative inlaid brass wire (called Cloisonné) on the ball surface. They look pretty, but if you dropped or chipped the ball, the metal wire may stick out. Get pure marble or metal ball. Also, most of them chime. I prefer them not, especially if you work in the office.
Chinese hand exercise balls