Programming. Learn C++. There are just so many C++ websites to name. Or website building, learn HTML, then PHP. Fun skills to learn.
If you want to sound smart, learn complex algebra.. that way you could talk about imaginary numbers and how real numbers are really numbers with no imaginary components. Or learn statistics: it's the most useful branch of mathematics in real life, especially in casinos.
Or learn to fight. Arguably, that's a bit difficult on the internet, because you'll have all sort of crap, and never figure out which martial arts is the best.
Or pixel art. Or photoshipping. Both cool skills, and something you can do on your computer. Can't really show off a hand sketch, but very easy to show off a pixelated one.
Learn body language. It's very easy, just grab like Body Language for Dummies. Also something you could find useful, especially when flirting.
On the subject of flirting, it's probably one of the coolest thing you could learn in your spare time. So, I'd suggest The Art of Seduction by Robert Greene. It's as close to a textbook as you can get on the subject, and very well written. If you don't like love, there's always 48 Laws of Power and The 33 Strategies of War by the same author. Ugly cover, laughable titles, but quality reading inside.