Academic Sit in on a class, preferably in your intended major - Nothing can give you more of a feel for the school than going to an actual class. You'll get an idea of class size, and a sense of what the students and instructors are like in action. If the currently enrolled students you meet recommend the college to you that's positive. Check out student bulletin boards, inquire about clubs, fraternities and sororities. See if you can easily find clubs and societies that you would like to belong to, or if the existing clubs are active or not.
Facilities Get a good look at all the facilities.
Visit libraries, sports fields, computer labs, etc. Since you won't be spending all your time studying, you should also look at what the school has to offer outside the classroom. Tour the campus without a guide and try to mingle with the student population inconspicuously. Take this opportunity to visit sites of interest not on the tour. Colleges are going to show you the parts of campus that they are most proud of, so get a look at the rest of the campus on your own for a more balanced experience.