Here is the definitive answer that you aren't able to find anywhere else:
DON'T WEAR A SUIT!!!!!!!!!
The schools that I visited: BU, BC, Harvard, MIT, Yale, and Stern.
In the classes that I sat in on at all of these schools, I would guess that 50% of the people are "very casual" (like t-shirts, jeans, hooded sweatshirts) and then the other half are a little less casual (jeans and polo, khakis and sweater). I went "business casual" and felt that I stuck out even dressed like that, but that's the standard. The only people wearing suits were the kids who happened to be interviewing that day who also wanted to sit in on a class that day.
I wouldn't plan on stopping by the admissions office. I know some books say it's a good idea, like, "And while you're visiting campus, be sure to pop into the admissions office and introduce yourself." This is horrible advice. It just looks like you're trying too hard and makes you look like you don't have the common sense not to come across like a car salesman. While it's true that it is good to "show a school that you're truly interested" in their program, the proper way to do this is by learning everything you can about their program, and then in subtle ways demonstrating to the ad-coms that you have a deep knowledge of their program in your essays and interviews.
I thought all my class visits were a lot of fun, I'm sure you'll feel the same, best of luck!