Congrats on your admits!
I have to confess, one of the first questions I asked my professors about my admitted student visits was "what on earth do I wear?" They basically told me to look respectable and be comfortable. The schools have already accepted you, so it's not the same kind of pressure as a pre-admit interview.
As for the visit, try to talk to as many current students as possible, and try to do so when faculty aren't around if you can. Ask them why they decided to go there, what the best parts of the program are, and what parts they wish they had known about before they accepted. Ask them about how well and how often the faculty members in the department interact. I'm not super-familiar with how MPP programs are set up, but if there's an independent capstone/thesis type project at the end, talk to students who are currently working on theirs to see what sort of support they're getting from faculty.
Also, see if you can meet one-on-one with a few of the faculty members you most want to work with, and ask them about what they're current interests are. In talking to faculty members, I sometimes found that they had moved beyond the work I thought they were still doing, and it was good to get a sense of the ideas they were presently exploring. You can also get a great sense of how the faculty work together during these conversations--if they refer to each other's work (and refer you to each other!), that's a good sign that the department's environment will be collegial.
Most of all, try to remember that, at this point in the game, you are the one choosing them. Have fun, and good luck!