How strong is your department? How many professors are there for that major? What do graduates from your major end up doing? How up to date are the labs? Do the professors seem engaged? Can you get AP/IB credits?
Campus life:
How are the dorms? How far away are the dorms? How is the dining halls? What is freshman orientation like? How do you get around town? How do you get home? Do they have/not have big sports as you like? Do they have clubs that you like? What can you do for fun? Do you like the size (Big or small) of the campus?
Other students:
Do they seem like “your people”? (whatever that is)…Are they academically focused like you? Do they look like they are having fun? Are the other potential class mates reasonable?
Financial:
This isn’t really campus visit, but part of making a decision…If deciding between two colleges, is this college a better value? Is it worth athe diffference in price?
It can be hard to visit every college where you are accepted, especially if the acceptances come in late March. We tried to figure out ahead of time when the accepted student days were (not as easy as it sounds) to see if there were conflicts so we could make plans. My D2 visited her top 3 that she got into at their accepted student days, and stayed overnight at all. 24 hours on campus was very revealing – her priorities changed as a result of the visits. #3 going in ended up #1 after the first two didn’t really put their best foot forward, and #3 did.
Think back as to why you applied in the first place, and make sure those things are still important to you. Spend time researching the major you are interested in at each one. Inevitably if you spend some time researching you will come up with questions. Look at the class descriptions in the course catalog. Look at the core curriculum. Check out the housing information and the weather. Greek life? Things to do on and off campus?
Strongly agree with @intparent - try and do an overnight at your top 2-3, and sit in on a class as well It can make a bit difference. Be prepared to discover that things that seemed important in the abstract are suddenly not so important.