Passive Wrist Stretches
These stretches will help you to create range of motion in your wrist flexion and extension by using the floor as assistance. If you have tons of wrist extension and zero flexion, your wrists aren't balanced, and your elbows are going to get pissed off in the long-term.
Never force any stretch and always listen to your body. If you notice that your wrists feel tender the next day after doing these, that's your sign to back off a bit. If they're okay then you can start to push a bit further the next time.
If even using the floor is painful, then step it back a bit. These exercises can be regressed by using the wall. Try doing them after a session with a lot of grip work. You'll notice forearm tightness practically disappear within minutes. As a side note, doing "shoulder rotations" in any of these positions also makes for a great warm-up addition.
Radial and Ulnar Deviation
Radial and ulnar deviation is the side-to-side motion of the wrists. It's neglected by almost everyone.
If you practice advanced hand balancing then this is absolutely crucial, and if you do overhead squats or snatch presses, this thumb-grab stretch is an awesome way to alleviate some of the pressure caused by heavy lifting.