When I was a math grad student, I technically could have gone through the process to get 1 or 2 CS courses approved to give credit toward my degree, but I never actually cared about the "credit" part, anyway. So, I did my regular course in the math department, taught for the math department, then hung out half the day in the CS department. I ended up auditing, and thoroughly half-assing, Algorithms 1, 2, and 3, Programming Languages, and Automata Theory. Still learned a lot, in spite of the half-assery.
Did your timetable/coursework schedule allow you to audit a meaningful number of external courses? I find that I can only allow myself to attend one additional course per semester.
I STUPIDLY started going to lectures on Computer Systems this year without doing my research first (I couldn't find the timetable for Compilers, and relied on being able to take it officially next year). Turns out I don't have enough (or any) background in C, and so I couldn't make sense of a lot of the lecture material. Oh well, at least now I'm learning C, and I still have the lecture recordings.
