I'll try and give you some advice, but keep in mind each school/campus visit will can be very, very different.
* If it's an organized tour, and you have multiple tour guide options - try and pick the one that has similar career objectives. If it's a small group, you can probably get a pretty good conversation going with him/her, opening doors to communication
after the tour. Make sure you keep track of the names of people you spoke with, as these can be useful during applications for certain schools.
* Ask questions. Don't be obnoxious, or ask questions that can be found in a 10 second google search, but if you have a question, don't be afraid to ask it. Most of the people involved in your visit will be student volunteers, as opposed to adcoms. So they won't be grading you or considering your conduct for you application (unless you do something really bad, such as the class visit discussion you referenced)
* Bring something to take notes with. You'll meet several people, and want to get their names. You'll also get to meet some fellow applicants who you may want to trade contact info with.
* Try and be aware of what's around you. Some schools will (obviously) try and take you to the nicest sections to try and sell their program to you. If you're standing at the elevators and someone opens a class door behind you, take a peek. See if it's comparable to the classroom they actually stick you in for the class visit portion. More importantly, you also want to observe how students interact in the hallways/classroom. Are they asking good questions? Are they actually paying attention, or are they typing away emails on a laptop? Do students know each other in the hallways, or all walk by each other silently (this may be more relevant for business schools that don't share a building with undergrads).
*If your visit doesn't include lunch, try and get in touch with a student in one of the groups you're interested in and see if they'd be free for lunch / coffee or something during the day. This is a great way to get more specific information into aspects of the school you're interested in.
I'm sure there's a lot more advice out there, a search would probably pull up some similar threads, but hopefully this gives you some useful pointers.