Ask HN: What are my options for cross discipline study after I graduate?

I'm a Junior in a Computer Engineering program. It's one of the largest majors in the University and I get a little annoyed that it doesn't leave any room for interesting yet unrelated classes. I would love to take an upper level Econ class as well as upper level Math classes. But the university is working hard to get me to graduate someday (I am already planning on graduating with 60% more credits than normal). I've resigned myself to the fact that if I want to study number theory in depth then it's not going to happen during my undergraduate. What are my options down the line? Have any of you gone back for a second degree "just because"? Is there any basic prep I should try to squeeze in now which would aid me in future study?