I brought a notepad but I never wrote anything on it...

make sure you have a pen. if you need a pen and don't have one you will feel like a tool. most schools give you pens and stuff though but just in case they don't i think you should bring one. So because you never know what you are going to get in an interviewer
I always bring a copy of my app. They may have lost this ( i have had this happen for a job interview so who knows with med school). I think this may be helpful too if your interviewer is super anal and wants to know dates that you did things. I also write down the dates that I started new activities that are not on AMCAS in case they ask. I also bring copies of my resume just in case they ask for it.
I also brought copies of my papers that I wrote for research in case they REALLY want to know a detail I can't remember. I just put it in a nice padfolio which also carries a nice looking pen. I write down any questions that I think of during the day about the school so I can make sure i get them answered. I just want to be super prepared but I am super crazy and I am sure I bring too much.