Ask HN: What cool stuff can I do with math?

I saw a guy walking with a book under his arm titled "Differential Equations and Linear Algebra". I thought, "I liked math until that semester in college when I played too many video games and dropped differential equations". So I decided I might study math again and in 10-20 years maybe I'll be an amateur expert on math. That's cool. But what can I do as an amateur expert in math? Maybe that time is better spent becoming an expert programmer, getting a CCIE, or any number of other practical endeavors. So what cool things can I do with math that might warrant a long term study? The best I can come up with is, it might be cool to know how to use statistics properly. The rest seems like mind candy, only interesting because I want to know how it works.