Don't fill your day with subject/welcome/finance formal "talk" sessions. They suck your time, and you just get stressed going from one to the other in time, get stuck in lecture theatres, and don't have time to soak in the surroundings/atmosphere etc. Just do 1 or 2 and spend the rest of the day touring the accommodation options, get a tour of the campus, go to the department itself and see if you can get a tour (there are usually tours of science labs etc if that's your course). Most importantly, talk to existing students, there are always loads around, giving tours, standing at info spots, giving directions, etc and they're usually happy to chat and answer questions! (obviously only the ones wearing the "welcome" t shirts etc - not the ones who aren't participating in the open days and just going to the library or whatever!)
Accommodation tours are important as standard/size/layout varies a lot between Unis and even within the Uni. Our DS's first choice Uni had very old/tatty/small accommodation which put him right off. He ended up at the Uni with the most modern/larger accommodation which was actually cheaper than the other! Of course, if your DS/DD is happy to sleep/live anywhere, then it's different, but it was important to our DS!
Once you've done a few, you'll see that the "talks" are very similar, so don't keep doing them at different Unis - they're mostly the same format, and tend not to give as much detailed information as on the Uni's website. Pointless to waste valuable/short open day time on detail that your DS/DD can research for themselves on the website - far better to spend the time talking to real people, touring the Uni/Accommodation, looking at the departments, etc - things that can't be done so well on line!
Our first was at our nearest Uni, which DS was "OK" about out, but we then we to several others and his preference changed with every one he went to, and ironically, his final decision was the closest, the one he went to first, as he decided none of the others were better overall even though all had pros and cons.