Just some stuff I do...
I've recently started doing some work with a 10lb sledge. Seems to be increasing grip strength a mite...
Holding as low as possible on the handle rotating back forth for reps. Arm bent 90 degrees.
Holding as low as possible on the handle raise up over head, then chop downward towards body center and stopping the fast movement with grip when hammer parallel to ground. Idea for both low as possible on hammer grip and hammer brought down fast as possible to stop it. Let it naturally chop down to center, this wedges hammer handle between pointer-finger area and lower opposite thumb part of hand. This focuses grip strength towards pointer-finger side of hand
Holding hammer with arm extended down at side, hammer to the rear. Swing
hammer head up behind you let it fall fast catch with grip. Again, grip as far down the handle as possible to stop the hammer at parallel to ground. Also, again, get hammer moving downward as fast as possible. This focus on grip strength towards pinky-side of hand.
Hammer seems to be working well, anecdotally still, but grip really feels stronger. This has been really therapeutic for my arms too. I find I can do more grip work since starting this. Kinda like doing weighted foot drills now lets me pound my feet harder.
Weighted wheel barrow actually works grip in a farmers walk fashion wonderfully, probably similar to dumbbells, but I think you can overload distance\time\weight easily and safely with wheel barrow. Becomes a real concentration exercise to tell your body not to let go. 20yds fine, 40yds fine, 41 yds omg my forearms are on fire, 60 yds where am i?
Double overhand everything as heavy as possible.
My gym has a trap shrugger with the spinning different sized grips. I over load this for holds. The fattest is like a ridiculous 3 inch diameter.
Wrist roller. 2 inch inside diameter PVC about a couple feet long with rope around its middle slide over a shoulder height'ish bar sleeve. Weight other sleeve with a couple 45 pounders and tie weight to rope. Roll up roll down repeat. Flipping the PVC back and forth lets you change roll direction. Focus just as intently on lowering as raising. Don't overdo this one.
I have used a lighter gripper to do tabbata timed grip sets, that is an interesting experience.
Hold gripper closed. Open and close elbow joint rotating hand. Therapeutic I find.
Gripper holds for time. 1 minute or more.