Eat lots of protein and carbs, and then start lifting weights consistently 3 times a week. I know you said don't suggest the gym, but weight training really is the best way to healthily gain weight, and the fastest, best way to gain muscle mass. Unless of course you want to get fat. Then just eat pizza all day and don't do anything active.
Food that's rich in protein and low in fat: Chicken, cottage cheese, fish, turkey, steak, eggs, peanut butter. Also, if you really want, get some protein powder and make a shake within 30 minutes of lifting. The cottage cheese is especially good to eat right before bed because it contains chemicals that prevents muscle breakdown.