It depends what you want to do with yourself: if you have a technical major, the mathematical requirements will become clearer. (For example, graph theory is much more important in computer science than in, say, physics.) For general knowledge, learn linear algebra and multivariable calculus. If you haven't already, read a book/take a class that's heavily proof-based -- for example, Rudin's Principles of Mathematical Analysis. Being comfortable with formal proof is necessary for a number of disciplines (not just math) and is, I think, good for your brain. (I think of learning to prove as a similar kind of transformative experience as learning to read -- except that nearly all of us learn to read and not so many of us learn to prove.)