I went to the ASD in January. You have to register and get a name tag, etc. so I think you can't bring anyone with you, but its pretty casual otherwise. They have a breakfast, some talks by various professors and departments, etc. Then you go on a tour with a student and attend some breakouts with professors who encourage you to ask tough questions about the school. Then there is a reception with more food, wine, beer, etc. where you get to talk to the students and professors more. My feeling was that everyone was really open and friendly, the professors are really smart, etc. A good experience overall, I think everyone who wants to go here should definitely attend and ask a lot of questions. But, I have to say the other accepted students were a little like sheep clustering together not wanting to speak up, etc.
As to what to wear, etc. I would say come as you are, but when I went it was a little cold in the rooms, etc.
Yes, you get free stuff. Other than food, you get a t-shirt, a little mouse that connects to the website, some pamphlets, etc. I wouldn't go for the free stuff, but I think the info you get is worth your time.