Many of the universities I've been looking at allow graduate certificates and/or PhD minors. Some degrees even require a minor. I've seen a few interesting ones for complexity science, cognitive science, and linguistics. Even though I want to get through a PhD in less than 7 years, I'm wondering if I should go completely crazy and do two minors? I should be able to earn one or two when I'm teaching at a university in the far future.
The reason I want to do this is because my interests stray from traditional computer science and even AI. Cognitive science is great on its own, but different programs focus on different aspects of it. I want to stay with computer science, but branch out and bring more to the usual PhD.
The complexity science focus comes from the fact that any 'real' AI (however you define it) will almost certainly be defined as a complex system. Cognitive science is another interest because I think the psychological aspects of human thinking and the philosophical theories of thought itself can give us many clues toward how an AI might be programmed. Linguistics comes in at a close third as we want an AI to be able to communicate with human beings.
I'm still looking at universities throughout the country and seeing if there are other minors that would be useful.