Aside from calculus, I'd say that it's worth going into:
• Algebra (linear for a very useful toolset, abstract for a challenge);
• Differential equations (numerical or not, depending on whether you enjoy programming);
• Stochastic analysis (very often useful to understand the concept of random variables in STEM, especially if you expect to use any kind of statistical analysis).
These subjects are a foundation for an insane wide range of professions, so hack away!
The math you will encounter as a math major will likely be much different than the math that you've seen in high school. In the latter, "math" far too often consists of algebraic manipulation without much attention to what's actually going on, and memorizing formulae and problem solving paradigms to regurgitate on exams. When you encounter advanced math classes in college such as Abstract Algebra and Real Analysis, you will need to read, write, and understand proofs as well as precise and at times unintuitive definitions. Thus, you'll actually have too understand WHY everything works the way it does.
Applied math is a little more what you're used to probably. At my school, applied math majors take optimization as well as calc based probability and statistics, and isn't nearly as heavy with proofs.
