One very interesting exercises for building grip strength is to grasp the opening of a large clay jar with each hand. As you get stronger, simply fill the jar with additional sand or rocks to increase weight to make the movement more difficult. This method is simple, but very effective, and martial artists have been using it for centuries.
This type of training had many names, for example, in Okinawan Karate, as a part of “Hojo Undo (supplementary exercises) they are referred to as Nigiri Game, or gripping jars. The fellow above is from 1906, and seems to have things pretty well taken care of in the grip department from using this exercise.