Lifting and stunting challenge bells is one of my absolute favorite ways to train for hand and grip strength. It is one of the core legs of the training model presented in Industrial Strength Grip Vol 2.
Many people have a misconception that training the hands would be a series of tiny isolated movements, but that is not the case at all. Rather than squeezing tennis balls or playing with rubber bands, I want you to try out thick handled explosive lifts, monster plate lifts, and pull on massive thick bars. There is no doubt, to get the best response in the body (for virtually ANY GOAL) compound joint movements with load and speed are the fast ticket.
Your grip training should be no different.