I will firstly caveat that in my experience the final part of the interview has never been a "make it or break it" scenario, and in 99% of cases by that point the interviewer will have already decided if he will pass you or not. However, the final questions can still be an important part of the interview. In my view there's two types of questions you should be asking:
1) Genuine information about the firm you could not find online: you should really take this as an opportunity to ask questions that you genuinely want to know the answer to. However, you shouldn't ask questions that can easily be answered with a google search, as this will make you seem unprepared/not that committed to the firm.
2) Personal questions about the interviewer: Chances are, the interviewer will have either introduced himself and given a short bio at the start of the interview, or you will have a received an interviewer bio before the interviewer. You should tailor your questions to them, and their experience.
Did they do a 6 month transfer to an office in Asia? Ask them about it and how the two offices differed. Was he/she a lawyer before doing an MBA? Ask about it and what made them switch to consulting.
People will much prefer talking about themselves than generic facts about the firm they work for. You are also much more likely to build a rapport with the interviewer this way - therefore, he is more likely to remember you and vouch for you if needed during the recruiting (of potentially, even when you join the firm).