What are some things that are important to know about colleges?
ex: Do you have an open curriculum?
Is this when visiting campus? Or at a college fair? Or are they coming to your high school? I am going to be honest, my kids had few questions that couldn’t be answered from the school websites.
When they come to your high school! Hi again, you’ve been very helpful in the past @intparent . 
So I admit, I would encourage my kids to have at least one question for the rep at those sessions. Just to be noticed as someone who actually seemed interested in the school. My kids went to a small high school, too (less than 100 students per graduating class, so sometimes only a few students attended the sessions). We would usually talk the day before about what they might ask. Sometimes just ask what the rep likes best about the college if nothing else seemed obvious (or if we hadn’t even talked about it, and nothing came up in the session they wanted to ask about). I do remember D2 asked the Carleton rep how studying abroad would work with the trimester schedule at Carleton when most colleges abroad are on something closer to a semester schedule. The rep apparently was impressed that my D had done her homework (um… I had wondered out loud about that to D2, but I guess she picked up on it). She did get into Carleton…
I think the point is to act interested at the session, and i do think it takes some pressure off the rep if someone has a question (so in that environment they appreciate it). If my kids interviewed for a college, usually they went in with at least a couple of questions – maybe something about double majoring or minoring, study abroad, or that standby about what the interviewer likes. But only one of my kids interviewed, the other did not (not great interview skills, more risk than it was worth).
At college fairs, they are usually zoos. Signing in on the sheet for a college is most important, maybe just introducing and saying that you are interested and taking their literature is probably enough there.
Sometimes my kids couldn’t attend a session at their school that they were interested in (test in a class or something). I would encourage them to stop by to introduce themselves before or after the session, explain that they had a conflict and couldn’t attend, but were definitely interested in the school. I know D1 did it, it is uncertain whether D2 (introvert) could bring herself to do it.
Try not to ask questions that are easily obtainable on the website.
You can’t go wrong with “What would you say makes unique?”
I agree with @intparent that if a rep comes to your school for a visit ask questions, act interested and put your best foot forward. The rep that comes to visit your school is likely the one reading your application. Adcoms are admitting people, not resume/stats, so portray yourself as likable, interesting and with something to offer.
Some other things to ask are questions about the application process at their school ie; do they offer ea, ed scea etc., how the applications are read, what types of kids do well at the school, could ask questions about the honors program is applicable, how the students interact with the city/town the school is located in. If you have a particular talent you may ask a question about clubs/classes that may pertain to that as that would be a nice way for the rep to learn a little but about you. Could also ask something like - I noticed from the tour/website that your college offers X, can you tell me a little more about that.
The important questions to ask is what is important to you. My S and D asked different questions as they went through the process. My D asked about research (she is a science kid) and if a non-music major can be in the orchestra (she plays violin). She also visited a trimester school and had specific questions about that. My S, a business major, asked questions about internships, study abroad etc.
And my D had a case where she could not attend a session at the HS given by a school she was interested in due to an exam in an AP class (teacher would not let her take exam another time). In that case she emailed the rep and let her know that she is interested in the college, had visited already, but could not attend the information session at the HS and that seemed to work out fine.