I don't really enjoy going to gym to build muscles or to put on some weight. I would love to find a challenge or competition which can help put on some weight or build muscles. For example to loose weight my friend ran a marathon. As a side effect he got healthy and lost 10 pounds.
Weight gain is the same as weight loss in the fact that is is all about sustaining a calorie controlled diet. One can gain weight purely through eating alone, how that food is processed and what final state it is on your body (fat or muscle) is determined by the amount and type of exercise you put it through as well as a the macro-nutritional (carb/fat/protein breakdown) profile of your diet.
Muscle is more weight-dense: 1 cube inch of muscle weighs more than 1 cube inch of fat. If you wish to continue eating at a maintenance of calories (at your TDEE) then adding in strength-based exercise will help you do re-composition.
If you wish to gain muscle then supplementing a sensible macro-nutrient breakdown with a surplus of calories (~500cal over your TDEE) and regularly (at least 3 times a week) doing strength-based exercise movements.
Common competitions that require a high Fat-Free Mass (lean and strong) and low(-er than most) Fat-Mass:
- Strongman Events
- Olympic Weightlifting
- Crossfit Open
- Wrestling Meets
Out of all of them, preparing for a strongman will lead you to the most weight gain due to the lack of weight-categories for a lot of events. Same (to a lesser extent) for crossfit. The other competitions are split down into weight classes which will pair you up with people of a similar weight and so you would train for your weight class.
To be able to compete in any of the above you are going to need to join a gym: strongman gym, crossfit box, power/oly gym. Weights are heavy and gyms are the perfect place to keep them, adverse weather is not the friend of the weightlifter.
As with all the above events, having a calorie/macro controlled diet is extremely important to ensure that you are kept lean, strong and have enough energy to train up to 5-6 times a week.
For more information on gaining weight, please read this post: I am underweight. How do I gain weight and muscle?