I majored in math. It was fun in a mind bending kind of way. The course of study covered:
Introduction to Calculus
Introduction to Analysis
Multivariable Calculus I (Linear approximations to functions in any dimension)
Multivariable Calculus II (Stoke's theorem)
Linear algebra
Abstract algebra
Complex Analysis
Real Analysis
Mathematical logic
Differential geometry
Galois theory
Elliptic Curves
I wrote my thesis on "descriptive set theory", which is the theory generated by considering extensions to the Borel sigma-algebra for a topology on a Polish metric space. So, it was in the intersection of topology, set theory, measure theory, and mathematical logic.
Also, I had a lot of freedom in designing my course of study, so I ended up taking a lot of logic courses from the philosophy department.