For POI's, I think I asked a lot about the lab, i.e., what projects were they working on/planning to work on, what I would be able to be involved in, what other students were working on, how often mentors meet with their students, etc. One "recommended" question I saw was to ask about mentoring style, though that question backfired on me. I also asked the POI and/or other interviewers what types of internships students from that program typically attend, how long students take to complete their degree, what types of clinical practica is available, whether there's collaboration across labs, etc. I asked students all of the questions I would really want to know about attending the program: living in that city, stipends, what there is to do, faculty/student dynamics, etc.
When I was preparing for interviews, I had come across this on a professor's webpage, which gives advice for "what to do once you get the interview" and includes a couple of pages of example questions you might ask:
http://www.unc.edu/~mjp1970/I Just Got an Interview for a Clinical Psychology Doctoral Program.pdf
I would just think about what you would truly want/need to know about a program to make a decision about attending and ask questions that would help you with that! Good luck!