Okay, two thoughts:
They're having you do your research presentation at night, after a full day of travel and dinner??? Yikes!!! If so, then I think Brixton gives excellent advice.
For the teaching plus meetings day, do wear a suit. For the teaching demo, think about some kind of a nice top or lightweight sweater that you could wear that would allow you to dump the jacket while you're in the classroom. I find a suit jacket confining, too warm, and too rigid-looking for actually teaching, and it sounds as if you may feel the same way.
Great advice from everyone - thanks! I think I will follow Brixton's advice for the travel/dinner/research presentation day (yikes indeed) and msparticularity's for the 2nd day. I didn't get the sense from the MLA interview that the committee would be offended if I didn't wear a suit, but I also don't want to violate any unwritten rules about campus visits.