I am studying physics and like math. I just finished courses in mathematical logic and probality theory and attended courses in calculus and linear algebra. I intend to learn theoretical informatics next semester. Any more suggestions for interesting mathematics? I started algebra this semester but dropped it because of my other commitments.
I agree with the combinatorics recommendation, you may find it listed as "Discrete Mathematics" as a computer science course. (There are other things besides combinatorics included in discrete math, but elementary combinatorics is the major part of the course.)
ADDED: You might consider reading about math some. The book, The Mathematical Experience, by Philip Davis and Reuben Hersh is a wide-ranging discussion of philosophy and applications of math. It is sort of like listening to a group of math majors with a couple of grad students sitting around talking about what they've found interesting.