Ask what the PI's mentoring style is like (are they very hands-off? are they usually available for questions or office hours?),
it's also important to ask what their current research projects are, you can also ask about what conferences they attend and what journals are students usually publishing in (so you can see 1. if they are successful at publishing, 2. the impact factor of journals they are published in, and 3. if the journals sound similar to, perhaps, what your research ideas could be published in.).
Ask the student questions about living in the certain town they are residing in (cost of living, recreational activities, etc.).
This is a great time to ask all of those important questions that could help determine whether this program would fit your needs for the next 5 years. But also, remember that this is an INTERVIEW. Every person you talk to in that department is part of the interview process and it WILL affect their decision, so be professional and courteous. Good luck!!