There are tons of great exercises for abs. I personally like grabbing a rope like you use for tricep extensions and hooking it up to the lat pull down tower or cable cross machine and doing standing crunches against it. As corny as it sounds, the ab wheel or a generic knockoff will do a lot. There’s also crunches on the bench while holding a weighted barbell for resistance, setting a kettle bell on your stomache and pushing it out with just abs, holding a dumbbell with one hand and doing standing oblique crunches, hanging leg raises, etc ad nauseum. Really depends on what you like and what your goals are.