I know this is a resurrected thread but I want to put in my two cents anyways.
I like high reps for grip work in the 10-20 range for most exercises. My forearms tend to respond better to it and and I think while 1rm might not grow rapidly doing it that way, higher repetitions keep my hands from feeling like death.
Levering (specifically side to side, 180 degree rotational, door knob turning style motions) is probably the best all around lower arm developer, especially for athletes. Use a 18" length of 1.5" pvc pipe and a couple of hoseclamps and make yourself a levering bar.
Thickbar work is second to me. I like pinch work and block weights fine but 3" pvc slips over the collars on your gyms easy curl bar and is way more easily loaded to bridge the gaps between a pinching 35lb plates and pinching 45lb plates.
Next are the toys. Pinch blocks, loadable hubs, rolling thunder, etc are all good fun, but they unless you have a specific goal or competition with them, they are variations of a theme. Use them as exercises variations if you have them or some extra money to buy them. But they're not necessary.
Grippers are fun, but imho unless you have a real definite ambition to close heavy ones, I would buy an adjustable like the ivanko supergripper (there's even a spring loaded torsion spring shaped gripper now I saw on amazon for like 80 bucks) and call it good. Lots of people find low reps, negatives, and timed holds work for them on grippers. I can close a 2.5 with a credit card set, and I use my grippers mostly for 10 rep sets and assistance.
Lastly are things like towel pullups... they're ok, but really I would personally rather get better at pullups. I've done a lot of rope pullups in my life and honestly, unless you're training for hand over hand events, your time is probably better spent on something else. Unless your grip is not the limiting factor on towel pullups you might be better served doing harder pullups and harder grip work.
I like farmers walks, but I'm not sure I'd rely on them as a grip builder but as event training, additional trap volume, or gpp depending on application.