The main objective is to have a good conversation and highlight your intellectual capacity and curiosity. Thus:
It is ok to ask:
It's not ok to ask:
Be prepared and good luck!
There are low-hanging fruits you can go after. The first is to ask about the interviewer - typically, they introduce themselves in the beginning and give you a little background. Try to remember a few things that interest you and that you think lend themselves to more conversation. "What exactly made you want to work in this field?" would be an example. Maybe you even have something in common that you can talk about ("Oh, you also did your studies at university XYZ?"). The second low-hanging fruit would be to talk about things you're really passionate about but maybe had a hard time finding information on. For example, you might be really passionate about working with NGOs, and can then ask if the interviewer ever worked on an NGO project and about how that works in the firm.
Lastly, there are some more generic questions, such as asking after the most interesting project the interviewer ever worked on. Or, if it's a more experienced interviewer, about how the job changed in his eyes. Less ideal, though, because it's far less personal.