An economics minor could also work if you're interested in game theory but you have to check the curriculum carefully. Usually you can mix mostly statistics (or econometrics) and game theory to get the econ credits.

I'd probably pick CS, math or psychology as a good combination with philosophy. Theoretical physics is probably interesting as well but I don't know much about it. Quantum physics seems almost impossible without dipping into philosophy from my pop-QP point of view.

I'm mostly interested in science theory/epistemology/ontology though and know fairly little about other branches of philosophy. No formal education but I've talked to philosophers on campus and it's usually very interesting. I'd agree that a major benefit seems to be a sharpened thought process in general.