An alternative grip exercise that can be done anywhere and avoids the injury risk of climbing, is to alternately make a fist and then open your hands as far as possible while spreading your fingers apart. Start slowly and gradually increase the speed and repetitions. Be sure to have at least one rest day between sessions (I actually think two rest days is better). After a couple of weeks, you should be able to do it extremely quickly for hundreds of reps until your forearms are completely fatigued. In each workout, try to open and close your hand more quickly than the previous workout. Make your fingers look like a blur of motion. It is surprising how completely fatiguing this can be after just a few minutes.