Angeliz, we have just started DD in the nursery of a local private school. TBH open days aren't that useful for deciding whether a school will be right for your child, you'll get a much better feel for the school on your school hours visit. I dragged DH round about 10 schools (I was determined to get it right) and think we've made the right decision. My suggestions are:
see how "guided" your visit is. We spent a good 20 mins plus in the nursery at DD's school. Although the head was showing us around he was happy for us to talk individually to teachers, NNEBs, assistants without hovering. DD showed a big interest in the puzzles and was invited to join in whilst we looked round. DH and I chatted to kids in the class and were impressed that they were happy to come and talk to us, as well as to talk to DD.
See what the settling in arrangements are;
Ask what the structure of the day is, can children have a rest if they get tired (for the nursery). Is there before/after school care?
Does the school do activities such as brownies and karate?
What's the situation on homework/reading?
TBH I found that i was most concerned with the nursery being "right" and the degree to which the nuresry integrated with the rest of the school. I really like the fact that some of the Y5 and 6 kids come to the Early Years playground and teach the little ones games like ring-of-roses, hopscotch and so on. The school also has "activity days" where the kids dress up according to a theme (pirates) and spend the day in their house groups rather than class groupss. They don't do any academic work but loads of art/drama/music/sport activites around the theme. That really impressed me as this school is geared towards getting the kids through their !11+
Check out the parents and be honest about whether you will feel comfortable mixing with them. We like DD's school because it really has a good mix of people, it's not dominated by the 4x4, gym mummies!
Finally, my mum's acid test, are the toilets ok? Seriously you can tell a lot from the state of the school loos!
Good luck, we're 100% delighted with our choice and there are some lovely schools around!