Really important advice: Don't come off as a gunner by overdressing. You'll just embarass yourself and feel stupid all day.
Unless the program specifically lists something like a swanky cocktail reception, you don't need to dress up at all and in fact should not dress up. Nice jeans and a sweater or collared shirt are fine. Basically, just keep it above a t-shirt and ripped jeans.
On the opposite end of the spectrum, do NOT overdress. Seriously. You should NOT wear a tie. You should NOT wear anything that requires dry cleaning. Do NOT wear anything that can be attached to the word "dress" (dress pants, dress shirt, etc.). You can bring a briefcase if it looks more like a messenger bag, but a backpack is fine.
Seriously! Every year there have been a couple admitted students who show up wearing a suit or something. It's just painful to watch.