Echo the above advice - make a plan to see a shortlist of colleges you have in mind, there should be students giving tours at each college; make sure that if they show you any accommodation you ask how typical it is - are they showing you the nicest most expensive third year room or the standard first year accommodation. Some colleges may have times when you can meet the director of studies in your subject - they are the person who usually would interview you and oversee your education if you get in so if you don't like them that is an important factor. if you can't meet them, try to talk to a student helper at the college who studies your subject and ask whether they feel they have been well taught by the supervisors of the college.
If you are already certain you will be applying to cambridge i would focus all your time at the open day on looking around colleges but if you are still deciding then it is worth going to a departmental talk on your subject.
must see places - kings college chapel if of course magnificent if full of tourists, a walk along the backs is lovely, the museums including the Fitzwilliam, the Zoology museum, the Sedgwick museum of earth sciences, and the Archaeology and Anthropology museum are great, and the gardens of Newnham college which are some of the most beautiful of the university in summer