I loved abstract algebra. If you've had a course requiring many proofs before, that might be the class that gives you a breather (math is not easy, but I think a first course in abstract algebra is not nearly so difficult as advanced E&M or topology). On the other hand if you have never seen upper division math, or written proofs, you may want to wait for the maths.
Or just drop one of the harder ones - four classes is much more manageable than five. I made the mistake of taking a similar semester, and while I succeeded and passed with A's, I think I almost died. I'm not sure if I ever want a 4.0 again, if it means that kind of stress... you don't have as much time to absorb the information as if you spread it out and really focus instead of jumping around between classes. Of course, my school didn't have an intermediate E&M class at all, so all of us were screwed in the advanced class, and we've only had maybe 5 people pass a topology class at all in the past 1-2 years...