I thought of questions ahead of time, but you might have questions just based upon your day there or the interview itself. I knew about each school from doing my secondaries and tweaking them a bit for each school.
Example: Going to my LECOM interview I knew that I wanted to know why they did prosections for parts of anatomy and how you could have a medical school in one building. Both of these things were negatives in my mind and the staff allowed me to understand both of these and turn them to positives. For UNECOM I wanted to know about housing in the summer since its such a vacation hot spot and rent went up during this time. For PCOM I wanted to know where to live and avoid high crime areas....
Key point, if you don't have any questions then don't ask any. But, if you have a question that will show your interest, illustrate that you were listening, or because you just plain wanted to know. Remember, doctors and admissions interviewers are just people, I promise.