Highly recommend wearing shoes that you can walk in comfortably - the tour was pretty long. (The new building is GORGEOUS, though.) Other than that, just enjoy and be yourself. Getting the invite is a big achievement in itself (they interview a fairly small proportion of their total applicants), so you should go in there feeling confident. And everyone there is so lovely and welcoming; very proud of their city and their school and wanting to share that with you, and very kind. They don't call it the "city of good neighbors" for nothing... The interviewers are on your side - they're looking to learn more so they can sell you to the committee. Know what you wrote on AMCAS and be ready to talk about it (extracurriculars/personal statement stuff was the bulk of both my interviews). The interviewers don't see your GPA or MCAT, but they see everything else on your application and that's what they'll be talking to you about. Also, prepare to have some ethics/professionalism scenarios brought up (nothing too crazy, just use your common sense).
Also don't stress if the answer is a WL. From what I've heard/seen on this thread, most people are WL'd post-interview and then the waitlist sees a lot of movement.
Wishing you the very best of luck!!