What are some different wrist strengthening exercises?

Light weights. Use light weights - as low as 1 lb., build over weeks to max 10 lbs. Do 10 "reps" (1 set) of wrist flexion -palm up, relaxed, curl your hand closed continuing as far toward the crook of your elbow as possible. Do to count of a slow 1, 2, 3 with 1, 2, 3 for reverse movement. Rest between sets for 3 sets. Palm down, gripping weight, bend wrist again toward crook of elbow for 3 sets of 10 .Reps.
Light weights. If you are able to lift without pain or strain, good wrist exercises involve gripping light weights and flexing the wrist up and down. Do not twist the wrist in an exercise; this puts too much strain on the wrist anatomy.