Start doing deadlifts and other heavy exercises without using any wraps on your hands.

Or the old timers used to pick up big 20kg discs in a pinch grip and see how long they could hold them.

Alternatively there are conventional wrist curls done with a barbell. YOur forarms rest on your thighs, when sitting down, and you hands dangle over the end of your kness. Lift your hands up and down. Depending on wether you have an overhand or underhand grip will determine if you work the top of your forearms or the underneath.

Hammer curls with a dumbbell also works the outer top edge of your forearms where it joins the bicep.