If my friend asked me, which sport should he do, I would recommend martial arts. They are really great kind of sport. First of all, doing martial arts can help my friend stay fit and healthy. Then nowadays it is a great idea to learn them, so you can protect yourself. Moreover, this kind of sport develope discipline. And also there are differen types of them: karate, taekwondo, aikido etc. So I would hardly recommend my friend doing martial arts.