It's hard to justify spending lots of time learning fundamentals while working on real projects. The kinds of things taught in college.<p>So, assuming one is a programmer, whatever one's weakest areas are: math, algorithms, assembly, operating systems (Linux), networking, distributed systems, electronics, machine learning, etc.<p>Or if one is already really solid in all fundamentals, and lacking more practical expertise, then a modern popular programming language like TS/JS, Go, Rust.<p>Or if you just want to learn for fun, I'd read recommend reading a lot of biographies of historical figures. And/or learning a foreign language.<p>And of course, learning how to cook your own food, exercise your body, and sleep well are probably more important than anything else you could possibly do for yourself.