It's unfortunate for me that I have not focused on health for a long time because of how I am parented and greater emphasis is given on studies. However, over a period of time I realised that some of the these activities are very helpful and enjoyable for ones good health and spirit.
* For flexibility I highly recommend learning Yoga as it flexes all muscles, improve posture and helps in other activities.
* Bodyweight workouts like chin ups, pull ups, push ups, core exercises, frog jumps, bar/tree hanging etc as it improves overall body strength.
* Swimming is a full body workout so definitely a healthy activity and also one can enjoy water activities at every age.
* Short sprinting instead of long marathons, long marathons tend to reduce your muscle so I would suggest not to go for long marathons on regular basis.
* Volleyball/ Soccer are very useful sports and enjoyable. Do not need to invest lot of money to have basic skills. Lot of Indians focus on cricket but its does not build good strength for the body where as soccer definitely helps in build stamina and lower body strength.
* Hiking is another great habit one can cultivate. I would highly encourage parents to take their kids at very young age for hiking as it is both enjoyable and fun activities. Mostly it will get away people from staying inside and into out doors, having a fresh breath of air.
* To gain strength and muscle I would highly recommend Weightlifting similar to olympics weightlifting rather than working out in a gym. Weightlifting is a full body workout and are more compound exercises than isolate exercises like working out in a gym. I would suggest as early as 14 years of age to get into weightlifting and make it as a habit.
If you dont want to do weightlifitng I would recommend Front and Back Squat, deadlifts, strict press, barbell rowing as the alternate workouts for weightlifting. Also i would discourage to focus on bench press, biceps,tricpes exercies as these muscles are not used that much. Have you ever seen athelets or soldiers or farmes having huge chest, or biceps or triceps. Majority of the workout plans these days are focus on these exercises which are not needed.
* I highly recommend Meditation is the most important activity and the most difficult form of activity as it's very difficult to calm your mind. Even though it is not a physical activity it helps your mind to calm down and relive of stress which is the major cause of health problems. I would encourage everyone to start practising at very young age.