Yoga classes are the way to go. I had a dodgy ankle that was constantly turning over, after a year of yoga I can wear heels again and it is totally stable. All of the postures help with core strength which is key to balance and stability. The postures that involve standing on one leg are good for strength too. You can do yoga at all levels, there is one class at our leisure centre that is very geared to older people with specific poses for helping knees and hips that are problematic. A good Hatha class with an understanding teacher is probably the best start.