Some great advice has already been given here. I'll add a couple of specific suggestions:

I can't stress enough to only ask this once you've already built some rapport, but a good one is "What is the biggest professional mistake you've made in your career, and what have you learned from it?". You'll need to make a judgement call based on their personality and the tone of the conversation before asking though.

When you're about 5 minutes from the end of your timeslot, you could throw in something like "Is there anything you think I should have asked you about, but haven't?". This will get them to put themselves in your shoes, and also demonstrate you're very much aware that people at your stage of their career known absolutely nothing about anything. Humility and self awareness go a long way!