What exercise can you do with a fractured wrist while in a brace?

Cardio. While you cannot lift weights, you can still participate in cardio activities, such as running, hiking or stationary cycling.
Up to surgeon. Certainly limited range of motion exercises of the fingers with the splint in place will be allowed under the supervision of a therapist almost immediately after surgery or immobilization. Above and beyond that, it depends on the stability of the reduction and serial x-ray examination and monitoring of the fracture status.