The interview day is essentially just the interview. There is a tour done by some of the MS-1s and an MS-2, but if you've gone to the open house, you'll have already seen everything. I hope your interview is early - in the group I interviewed with, this one girl had to wait until 3:30 to interview! And she arrived at 11!
For me, the interview was a kind of high-stress compared to my other experiences elsewhere. But that's because they were focusing on weaknesses in my application. But there are many different types of interviewing tactics - it's important to remain calm, composed, and smiling. You're there for a reason - they like you!
Good luck.