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.