The side plank is a core exercise: it trains your lower-back, abdominal, and hip muscles. Great for preventing lower back pain. And no gym or equipment involved! Many ways of doing it, here are some instructions on how to do a common one.
Resistance training, a key component of my weight loss/maintenance, builds muscle, strengthens bones and improves metabolism. It has tremendous benefits for anyone of any age. (Studies highlight the benefits for older women in particular).