If asked to recommend an activity that will improve one's physical and mental health, I would suggest frequent, solitary walks, preferably in a natural setting. The physical benefits are most obvious, especially related to conditioning, cardiovascular health and weight control, but many will find that walking also improves the clinical response and symptoms of established medical conditions.
With regard to mental health, solitary walks offer relaxation, reduce stress and provide down time to sort through personal problems. In addition, such walks often augment creativity (artistic or practical in nature), something to which I can personally attest; indeed, most of my writing topics are conceived during walks (this post included).
No doubt, some readers will scoff at this simple recommendation but I encourage them to give it a try. Over time, the solitary walks will prove to be both enjoyable and beneficial. Of course, as the ads for other therapeutic programs advise, results may vary.
