I second badger's recommendation of the Princeton Companion. In fact, I expect that reading it might give you some ideas of your own as to what math to study.

You say below that you are interested in "fundamental" mathematics. Based purely on that, I would recommend abstract algebra, number theory, or some sort of course in proofs.

Also, this might seem obvious, but go talk to a math professor if at all possible. Much of the answer to this question depends on what the specific courses open to you are.