I did a couple of open days with dd last year and she is going to several in the next few weeks & autumn, either with me, friends or cousin. From last year's experience, what I've encouraged her to do this year is to read the prospectus and course spec for each in detail, and also think about what she wants from university life, then make a list of what else she needs to find out. So the lists for each uni are things like:
Specific questions about course content, structure or options for field trips/year abroad (including costs), etc
Current interests/hobbies/sport - are there clubs/facilities for continuing these?
Things she might like to do (e.g. learn a language, take up a sport, etc) - would these be possible?
Where do 2nd/3rd years live - is there a specific neighbourhood or are they spread around the town(s)? Costs, social life in those areas, distance and transport to uni?
Different cultures in different residences e.g. are some more sporty, social, public school, etc?
After last year, dd decided the main things she needed to see were the university department (obviously), sports centre, accommodation, journey to city/town centre, and general look around to get a feel for the campus. She's also booked onto the relevant subject talk.
The best source of info we found last year was chatting to current students about everything, whether it's the course itself or social/student life issues, why they chose this university, what they like/dislike.
Like others, dd has ruled out London due to high cost of accommodation & also 2nd/3rd year students being so widely dispersed around the city and often quite a distance from university.