Come on guys, you have to think broader than some of the answers given. Why learn a purely physical skill because in the real world chances are you won't have the athleticism to do those skills? Martial arts takes years of practice to just get the muscles strong and good enough to do some things. You have to do something that requires your brain. Think of something that takes people years of studying to achieve. Law school, Medical school, Sales/Negotiation, Physics... Why learn a programming language? They get outdated and you will ahve to learn a new one. Calculators and brute mathematics isn't a learned trait as much as a trait by how your brain is composed. Autistic and Idiot Savants are some of the best calculators or photographic memory people out there.
I would choose to learn All Known Medical skills and knowledge because one, it makes a great difference in a person's life (you can literally save lives), and two, not many people out there could do it, so there is a need for people like this.