One excellent grip training/hand rehab exercise is the rice dig.
All a rice dig consists of is sticking your hands into a large tub of uncooked rice and swirling them around, grabbing a handful of rice and squeezing, or otherwise moving your hands against the rice. Then you withdraw them, partly or completely, and do another repetition.
It is a very simple exercise, although it requires a specialized tool - a big bucket of rice you won't ever want to cook and eat. It's deceptively easy at first, but it's rapidly tiring. As your hands get tired, it gets harder and hard to keep the fingers together. The rice forces your fingers apart as you push down. It gets harder to grip the rice, and your fingers strain against the rice as you move them around. It provides light resistance, but just enough - more than water, much less than a gripper or thick bar exercise.
Here are two descriptions of rice digs:
Joe DeFranco on rice digs.
Dave Tate on rice digs (T-Nation, so not W/F safe).
And a nice video about them, as well, on Youtube.
It's great for strengthening your hands and for rehab as well.