One of my senior friends swears by aqua fitness. She says it keeps her bones and joints going, is not too strenuous, in fact is soothing and relaxing, and she enjoys the social aspect of it as well.
I've heard wonder stories about yoga, and have met much older people who have been doing it for years and have the flexibility of people a fraction of their age. Yoga focuses on several areas which benefit general health and well-being including flexibility, posture and deep breathing.