I think you don't have to necessarily ask something that leads to a discussion. When I was interviewing people, I saw this question as an opportunity for them to ask me anything they found relevant to form THEIR decision. I wasn't expecting them to necessarily ask something smart or sophisticated - i.e. these questions did not affect my recommendation to hire the applicant.

A typical question was about how their typical working day would look like. I think this is a very important question from the applicant's perspective - because even if you know what the subject is about and what the team in general is dealing with, this doesn't tell you if you end up doing Power Points for the first months or are directly being trained to take over parts of the daily business. Furthermore it gives you an idea of the percentage of your job that consists of pure routine duties.

You could also ask about the background of your potential future colleagues. If you have a strong technical background and studied science it is good for you to know if you will work with like-minded people or not. Or would you want to be the only physicist in a group of business graduates?

Finally, do ask about your development perspectives - what kind of trainings do they offer etc.