I would agree with you that swimming is good exercise for older people, but horseback riding probably isn't the best idea for most seniors. It does use several muscle groups and can burn calories (as any rider knows, you must constantly adjust your body to remain upright and in control). However, riding is a sport that requires you to already be in pretty good shape; the demands of riding can stress arthritic joints in the hands, knees and shoulders, worsen back pain, and aggravate prostate problems in men. I wouldn't suggest it as a beginning form of exercise. However, seniors who are accustomed to riding can continue to enjoy significant exercise benefits for as long as they're comfortable on horseback.
At any age, an exercise program should ideally contain three elements: aerobic activity (such as walking, swimming or biking) for cardiovascular fitness, resistance training to increase muscle strength (which declines by about 15 percent per decade during one's 60s and 70s) and exercises to increase flexibility and balance, which can help prevent falls as you age.
I suggest walking as an ideal aerobic exercise - we all know how to do it, and you don't need any special equipment other than a good pair of shoes. Seniors who have joint problems that could be aggravated by walking can get an equally good workout from swimming or talking water aerobics classes.
Resistance or strength training for those over 65 can be done with free weights or at a gym on weight machines. The American College of Sports Medicine recommends that seniors do a resistance training workout at least two days a week (but no more than four) with 48 hours of rest between sessions. Be sure to get instruction at a gym or from a personal trainer to learn the correct way to do these exercises. The idea is to work the muscles of the chest, shoulders, arms, back, abdomen and legs by doing one or two exercises per muscle group. As this gets easier, you can increase either the number of times you repeat each movement or the amount of weight you're working with.
For flexibility, I recommend stretching classes, yoga or Pilates, a conditioning system that increases both core strength and flexibility.
I also highly recommend tai chi, sometimes called "Chinese shadow boxing." Tai chi is a formal series of flowing, graceful, movements performed slowly, and designed to harmonize the circulation of energy (chi) around the body. Like yoga, tai chi is a good method of stress reduction and relaxation, and it also promotes flexibility, balance, and good body awareness. It is beautiful to watch and enjoyable to practice and has proven to be particularly good for the elderly, because it reduces risk of injury from falls.
Andrew Weil, M.D.