DS went to several open days last year and this year. He did a similar thing each time:
Before the open day he had already read the information on the website, which had a lot about course and accommodation.
At the open day, he went to an admissions talk, which answered any other questions you could think of (and many you hadn't). There was one university where neither the website nor the admissions talk mentioned how they offer if 4 A-levels, so he asked at the end of the admissions talk.
He also looked around the department and labs (applying for science course).
Afterwards, when narrowing down his shortlist, he wanted to find out a specific question about adjustments for a disability, so he emailed the ones on the shortlist and compared the answers.
If there's time, it can be useful to find out where the accommodation is and how easy it is to get to from lectures, but don't base decisions on liking or disliking the room(s) you view on an open day. All universities have a whole range of rooms available from basic to luxury, from tired-looking to recently refurbished. If they choose the university as their firm choice (and occasionally if they choose it as their insurance choice) they can express a preference about which accommodation they'd like, but it doesn't mean they'll be allocated that one.