I liked to put them on the spot. I asked what they thought some other schools do better, or what they think their program needs to improve upon. As long as I phrased it correctly it led to good conversation. You probably want to be clicking on some level before asking this however.
I recommend asking open ended questions in order to build rapport. When they say, do you have any questions, it's an invitation to take the conversation in direction you are curious about. It's not an opportunity to grill them and collect discrete data.