Subject talks and demo lectures have their place at Open Days but my DC, as solo attendees, felt that the informal chats with current students and dept academics, often in the dept reception area or similar, were the best bits from their perspective - so these also need to be organised to encourage prospective students feel they have space to ask questions in their own way and time. Of the two Open Days I did attend, I could see that the layouts, and how the current students would draw in the prospective students, all aided that; so I think many unis are trying to choreograph events sensibly.
Things may not have worked as smoothly at the U of Bath Open Day for one of my DC (as flagged in an earlier post) because each department had a stand in a single large hall, a bit like a trade fair. My DC felt departments had less space to 'disperse' conversation groups and particularly assertive parents were prone to busting into existing one-to-one conversations.
Finally, to pick up on the point made by some posters about many journeys not really being feasible by public transport, I can well see this the case in some areas, particularly deeply rural and/or with the need to use a slow, infrequent service before joining the main network.
However, my DC attended several Open Days at unis, like U of Bath, popular with SE students, though they travelled from a region much further way and also contended with cross-London changes (as many Home Counties students would have to factor in). Very early starts were required and some morning sessions had to be substituted with afternoon ones or catch-up chats arranged with academic staff. It was a hassle for them but also very good for building up general experience. So, where there is a will, there is a way.
The solo attendee approach is not for everyone but, having discussed this thread with my DC, the bottom line is they said they got more out of the Open Days without me there but still felt I had been helpful in other ways. So there can be different routes to the same destination which, in this case, is about equipping DC with the skills to make informed decisions.