Like so many great things in life the answer is books, big piles of books.
You can train your grip strength by gripping anything really but wider is better because you’re using real squeezing force generated by muscles rather than curling something in your fingers and letting gravity play a part.
Really almost all exercise is about working against gravity as obtrusively as you can, now that I think about it. So be difficult in your battle against gravity, taunt it.
Look at me gravity, I’m doing pull ups, temporarily escaping your hold on my earthly body. Oh gravity you’d like to pull this stack of books from my hand but my mighty forearms prevent you, ha ha.
It’s okay, be a weirdo. It’ll make training better.