I just visited today as a day visitor as an accepted appointee, just dress casual, and bring what you would think you would need for any overnight stay.
Just through past experience, I would suggest a sharp haircut and business casual dress ie…Khaki Pants and Collared Polo style shirt. I would also take athletic shoes, shorts and T-shirt to relax or “Play” in.
Rule #1 of what not to do as a plebe: Don't be a foolish, idiotic, cocky "B"
Bring some athletic gear as well as kahkis and a polo shirt (neat, comfortable, clothes). The athletic gear will come in handy to go to the gym or whatever in the evening.