Are you signed up to the parents’ and supporters’ newsletter? That will give you info about, eg, college open days outside of the university open days, subject tasters, etc, which are useful to know about.
https://www.undergraduate.study.cam.ac.uk/parents/parents-newsletter
I second staying in an actual college just outside of term time (also if you need to stay in Cambridge the night before the official open days, so that you can start early). That way you get to experience a few college rooms, have free access to a college, and test out their food… There will be some students still around shortly after the end of term, in any event. Some of the further out colleges closer to the computer science labs even have parking, so you don’t have to park and ride to your accommodation; otherwise coming in on the train is a good idea. Outside of term time, you are likely to be allowed into quite a few colleges to look round, even if officially “closed,” if you say you are a prospective student (definitely not King’s, though), but I really don’t think the porters would be so obliging during term time, for security and privacy reasons. As a result, imvho, you won’t get a particularly good idea during term time what it’s like being a student, only what it’s like to be a non-student locked out of great swathes of central Cambridge and threatened from every direction by bicycles!
Just wandering around Cambridge, visiting museums, eating out, doing general touristy things will give you a feel of it and, most importantly, give you an idea of the geography before the open days, so you don’t waste time trying to work out where to go next. I definitely warmed to Cambridge once I felt I knew my way around a bit, had experienced a few friendly porters, visited a few museums and touristy sites, the botanic gardens, tried punting, etc (before that, I was a bit dubious about its charms and much preferred Oxford). I think this little fold-out map is great for seeing how the centre of the City all fits together, where the one-way systems are, etc, too:
https://harpercollins.co.uk/products/cambridge-a-z-pocket-street-map?variant=40348044558414