People usually think of exercise as something that is good for your health, but it can also help you save money and stay out of debt. Exercise keeps your body in good shape and helps avoid a number of expensive health related problems. Some health insurance companies are starting to charge extra fees for obese members or give discounts for those who try to stay in shape.
Exercising doesn't have to be complex, require expensive equipment or special coaching. Just walking a few miles a few days a week will have significant health benefits for most of the population. Gym memberships are always an option for people wanting to do more intense training, but they aren't always a good deal.
The best exercise is exercise you actually do on a regular basis. Be realistic about what you can actually fit into your schedule and try hard to maintain consistency over time.