I think a skill that can be both challenging and rewarding is learning a new language. This is something that has real-world applications and plus, knowing how to speak in another tongue is dead sexy. I'm studying abroad the semester after this upcoming one, so I'm learning Italian. My parents both speak it fluently but I'm also using this really helpful site Duolingo and slowly but surely my skills have been coming along.
Around 2006, I interviewed a guy for my magazine publishing class, who had been working with fashion in some capacity for a long, long time. He told me that if you have any interest at all in working for a fashion magazine, knowing Italian is a huge leg up. Journalism isn't paying so well these days, but still, having another language is pretty useful. While the ability to speak another language is often seen as sexy, there are plenty of really unsexy languages out there!