I'm a 4 x British Grip Strength Champion, 2 x European Grip Strength Champion and multiple British, European and World record holder in hand, finger and grip strength. There's probably 50+ exercises. Heck, I've a stroke rehab book written by a friend of mine (Dr To Balchin) which has 40+ pages on hand and grip exercises and rehab.
Climbers often train on what's called a finger board. Some gyms have these bolted to the wall somewhere if you're real lucky. If not here are two very simple moves you can do at the end of a workout
Hangs: both with the hand closed or open from a chinning bar (do not drop the shoulders) or on the frame of a power rack (do not use the thumb in this variation and it'll be like some climbing holds)
Proper old school dumbbell or barbell wrists curls or, more accurately finger rolls. Instead of a wrist flexation just let the hand open and close with the bar in it. You can do this with the bar behind your back.
I've sh*t load more than that if you want some more.
Here's another - get yourself a piece of 3 x 3-inch square timber. Get a nice half inch 'eye bolt' through it and hang weights from that. Curl it, carry it, hang from it. Use thinner blocks when you get stronger. Work until you're doing bodyweight on it.