I liked to ask about student life, since I knew that most schools enjoy their bragging rights about the normal stuff (boards, getting patients, preparing their students), and they wouldn't be likely to saying anything BAD about those things. So I would ask things like
--What's the dynamic like between students and faculty? Is it super formal, casual...do professors call each other "doctor" or use first names...etc.
--What are some interesting things to do in this city, and what have you seen students get involved in?
--How many professors are still practicing while they teach? (this gives you a good idea of whether profs are burnt out and hate private practice, or teach on top of practicing)
--What has changed recently in the curriculum? (cutting part of gross anatomy...dropping/adding courses...changes in clinics. Gives you a good idea of what their goals are)
--What are some committees or student groups that I could get involved in? Have you seen these groups make a difference in student life and administration?
When you're talking to students, you may want to ask more random, practical questions like what they do to party, how parking is, good places to eat....apts with cheap rent...that sort of thing.
Remember, especially if the school is a plane trip away, this may be the only visit you get before you make a decision, so try to get as much information as possible! (I know, it sucks when you're nervous...)