I think combinatorics -- both enumerative and graph theory -- are likely to prove very useful, perhaps the most important thing you could study. I suspect that

I think combinatorics -- both enumerative and graph theory -- are likely to prove very useful, perhaps the most important thing you could study. I suspect that differential equations could be left out if you choose. IMHO the subject is boring, but naturally others disagree. I suspect it will be indirectly helpful to get practice writing proofs. I imagine some of these courses already involve proof-writing. Finally, I also believe it will be helpful if you see some mathematical abstraction. I think the best place for this is abstract algebra (= "modern algebra"), and in particular group theory. Here is one fact to convince you that group theory is cool. It turns out that there are 43,252,003,274,489,856,000 positions into which you can manipulate a Rubik's cube. But suppose that you are allowed to take apart the puzzle and then put the pieces back together any which way. Then there are 519,024,039,293,878,272,000 positions - exactly 12 times as many. Why is the ratio of these an integer? Study group theory, and you will have a very good intuition for questions like that. Probably not something you will need directly, but I imagine it will sharpen your mind in the direction you'd like to go in. Good luck to you!