Try weightlifting.

Lean muscle is generally regarded as being healthy weight, as opposed to non-healthy weight, generally fat. To increase weight, you need a supply of the building blocks for the body, i.e. protein for the muscle. Sources of protein include meat, fish, meat alternatives such as Quorn and tofu, and to a lesser extent dairy produce such as milk and eggs. Note that red meats and dairy products are often relatively high in fat content as well, so if you don't want to increase your fat content, stick to fish, meat alternatives and whiter meats (poultry). However, a too low fat content is also unhealthy, so although this is rare in the Western world, aside from eating disorders, it is possible that people with certain malabsorption problems can be underweight. A supply of carbohydrates is essential to supply the muscles with energy, as even at rest your muscular system is using energy to maintain posture. Simple carbs such as found in sweets should be avoided unless you plan on exercising soon after, since the sudden increase in blood glucose causes an insulin spike and a large proportion of the carbs are stored as fats. However, complex carbohyrdates are excellent building blocks for healthy muscle, so things like wholemeal bread, pasta and porridge are very good foods that fill and don't cause you to feel hungry again an hour later. To increase your muscle mass quicker, you should try to stress your muscles. This leads them to repair themselves and grow, and the usual way is by a weights routine. If you aren't interested in looking like a bodybuilder, a moderate weights routine is still helpful. Use free weights to target large muscle groups rather than machines that isolate individual muscles. If you are serious about packing on muscle, then increase your weights slowly to find your safe threshold and aim to carefuly increase your strength by increasing weight carefully and slowly over time, ideally under supervision. You can also use protein supplements such as shakes and bars. However, if you are thinking of this option, use only reputable manufacturers such as www.maximuscle.com and choose the right option for you, as some mixtures target fat bruning, some target protein only, some include carbs and so on.