Piano playing IS a finger dexterity exercise :)

Be careful with flexibility exercises on the fingers. There's so many little things in there that can get damaged if you get too carried away. I've been doing Alexander Technique to improve my flexibility and range of motion in one hand -- I lost a lot of my range due to RSI. I've noticed the exercises are having the side effect of improving my dexterity, particularly in the hand that isn't damaged.

The book "Conquering Carpal Tunnel Syndrome" has a bunch of great hand and wrist exercises. Just be sure to do them very gently (and ignore the carpal tunnel bits.)