I'll be interviewing some people tomorrow as well, except it will be a group interview so I won't be the only one asking questions.
One question that I think is really great to ask in interviews is this:
What do you do in your professional and personal life to relieve stress?
This gives you the opportunity to hear the person talk about their personal life, get a sense for who they are, how they might react to a stressful situation, and what calms them. I think it says a lot about a person.