Every interview I've been to has offered a room in or near the admissions office to keep luggage during the day, it's expected that interviewees travel to the interview.
Bring a portfolio, not your huge briefcase (you will take a walking tour on most of your interviews, I find this usually involves a lot of stairs and shortcuts to make a single loop - especially at NYU, Columbia, and AECOM, and long, long hallways if you're in the Northwest at OHSU or UW) because like the interminable structure of this sentence, you'll have to carry it throughout the day and on this tour, and the briefcase will feel as awkward as it looks.
In your portfolio, keep a couple 2x2 pictures of yourself in case you forgot to submit them earlier, a couple copies of your personal statement (some interviewers ask for it), a copy of your complete AMCAS, a copy of your secondary for that school, a pad of paper for notes (it helps when trying to ecided between schools to have your first impressions on paper) and for questions to ask your interviewer (I've had moments where I went blank at that point), and a pen with ink. I took shuttle flights for my nearby interviews (BOS to LGA or DCA) and only traveled with the portfolio, so my boarding passes were in it as well. Not having a lot baggage to deal with took a lot of stress out of the process.