I attended a talk by a physical therapist and she explained the importance of working on the core muscles of the body, those of the trunk below the arms and down to and including the hips. I looked at books on exercise for the muscles involved and the one that seemed best was Peggy Brill and Gerald S. Couzens, "Fifteen Minutes that Change Your Life". It is clear and quick and seems useful for keeping the body in good pliable shape. Later, I was introduced to "3 Minutes a Day to a Pain-Free Life" by Weisberg and Shink. They advocate moving the spine in six ways each day. There is plenty of similarity between the two sources but naturally, you can do more in 15 minutes than you can in three. The links below show the recommended moves. You may want to modify or omit what seems inappropriate for your body or what your doctor does not recommend.
Squat
Wide-legged stance and wide-legged touch to floor
