I always try to ask questions about the culture/atmosphere of the school and lab. My goal is to find out how well I would fit in the program.
Some ideas:
Why did you choose the program?
Who do you regularly collaborate with at this school?
What is the atmosphere like among students? (need to know if they're all cut throat and super competitive or work happily with one another)
What would you change about the program?
What groups/seminars should I participate in once I enroll?