as for attire (this is psych): pre interview social. It can be a bit more casual. Lot of people would wear something like a nice black jean or something (or any dark pant, but not ur pant suit) and a bottom up or nice sweater. That would fine.
Post - you'll usually be in your interview attire, although some schools may give you some time to change/go back to the hotel/accommodation.
DO NOT GET DRUNK. Also yes, be yourself, but I've also had an experience where an applicant was saying 'borderline' racist things and got rejected because of that (it was just like a wtf moment).
DO mingle with the grad students, particularly those from your lab of interest. They can potentially vouch for you! Also ask some questions about the program and so on.