What courses go well with a mathematics degree?

This is a great question, and I think that ultimately comes down to what interests you.

I got a Mathematics degree, and paired it with a Business minor for a variety of reasons. First, I had already taken many of the courses so this made sense to finish out the remaining requirements, and secondly, I personally felt that would suite me better in my career. I ultimately know that everything revolves around business and if I could understand that side, and have my mathematics major to pair, I could do anything I wanted in a career.

What interests you and what do you see yourself doing for the rest of your life? What is your ultimate goal?

I may even suggest reading "101 Careers in Mathematics" to have an understanding of the endless opportunities as well as seeing what interests you and what courses would line you up for a career where your interest are.

Physics (or astrophysics) is math-heavy and pairs very well with a mathematics major. The coursework can be pretty heavy at times, but the feedback between the purely abstract math and the applications in physics can also make parts of it easier. I took that route and have no regrets about it, even though I chose to leave both fields for my career.

I suspect chemistry (or physical or computational chemistry if that's a separate major) would also work well, as would various Earth sciences.

You'll be assigned a faculty advisor in your main department (math) when you get to college; definitely talk to him or her about it, too, since they'll have direct and recent experience with multiple students who double-majored in various combos.