Yoga classes are excellent. I've got an MLB club's off-season training manual for their minor leaguers and it specifically states to enroll in a Bikram Yoga class, not just for catchers, but for all players.
Another excellent activity that will help your flexibility and balance, focus, etc. is to take a martial arts class. A year of that will give many benefits that will help you on the diamond, as well as off. Finally, focus on stretching AFTER you've worked out. Warm muscles stretch better than cold ones. Don't overdo it or rush it, gaining significant flexibility takes time.