You have narrowed it down to two or three acceptances and have decided that you want to visit the campuses before you make your final decision. My first suggestion is that you try and plan your trip sooner rather than later. Not only because the law school will want your deposit in the upcoming weeks but because they will soon be preparing for finals and will not allow outsiders to sit in on a class. You want to be able to see a class in action and have an opportunity to talk with both faculty and students before they go into finals hibernation mode.
Once you are on campus, you want to be sure to check out a number of places, keeping in mind what is important to you. What does this mean? Are you moving to a new area and will need to create a new social network? Are you already involved in the community and just want a degree? Are you relocating with a family? Your circumstances determine what is important to you and what you should be looking for in a school.
Before your trip, arrange with admissions to sit in on a class or two. Is it a lecture or interactive style of teaching? Are students engaged in the discussion? Observe the students, watch for interactions with the faculty.
Hang out in the library. I am less concerned with the collection of books and internet materials and more concerned with whether it is a place where you could study. Are there students utilizing it? If you want to meet people, you want a campus where students stay for the day versus just attending class and heading home.
Talk to students and faculty. Ask student what they like about the school, why they ultimately chose it, and about the general atmosphere. Ask faculty about their teaching style, do they work with students outside of the classroom, and if there is support for you particular areas of interest.
Visit the career center. Do they have internships and job opportunities in both the location and field that you are considering? Do they have an active alumni network? Do they offer career development workshops?
While you are at the law school, be sure to explore the surrounding community to determine if it is the right place for you and check out the housing options if you will need them.
Remember that not only are you deciding on a law school, you are choosing the place where you are going to live for at least the next three years of your life. Use the campus visit to decide which school meets the most of your needs.