You can always do a double-major.
I went to school at UIUC which ranks in the top five for CS and top 15-20 for Math, and we offered a Math/CS (Liberal Arts) Major. Some students also chose to double major in CS (Engineering) and Math.
CS (Engineering) at a typical research university will typically have about 80 credit hours of required courses: about 15 in Chemistry and Physics, 20-25 in Math, and around 40-45 in Comp Sci or EE.
Finance, on the other hand, has about 40-50 credit hours of required courses with about 5-10 in Math. Finance will limit your grad school options more than an an engineering discipline.
One other interesting route, IMHO, might be Applied Math and Actuarial Science. AS gives you the perfect backup plan- you can work at an insurance company for a few years if you don't get into grad school right away, and actuaries tend to have less cyclical job markets than finance majors.