Good workout
While I do recommend a good weight training program it is very obvious that martial arts provide an excellent cardiovascular workout. It is also an excellent way to release tension. A common denominator to most styles is kickboxing. The techniques of karate, taekwondo and kung fu can all be adapted into a sparring form that employs fists and feet. While I do recommend sparring with more advanced techniques at a later stage kick boxing is usually the safest way to start free sparring.