Aside from basic math (calculus, linear algebra, probability, ODE, all with proofs), take courses in topics that feel interesting to you just by themselves. Don’t count on things you learn being actually useful in real life, and accordingly don’t try to prioritize courses by that metric. You’ll learn what you need for your job by yourself or be taught at the job anyway, so instead spend this time building up an inventory of things to draw upon for useful metaphors. It’s easier to learn what’s intrinsically interesting so you’ll end up learning more. For real world skills, do some academic research projects and industry internships.
Aside from basic math (calculus, linear algebra, probability, ODE, all with proofs), take courses in topics that feel interesting to you just by themselves. Don’t count on things you learn being actually useful in real life, and accordingly don’t try to prioritize courses by that metric. You’ll learn what you need for your job by yourself or be taught at the job anyway, so instead spend this time building up an inventory of things to draw upon for useful metaphors. It’s easier to learn what’s intrinsically interesting so you’ll end up learning more. For real world skills, do some academic research projects and industry internships.