Chloerose, I would suggest getting a firm grasp on why you want to go into nursing, why you chose your specialty, why you want to go to Penn specifically and I'd brush up on current events in your field. I definitely did not feel like my interviewer was trying to trick me with obscure questions or anything. I wish I could remember some specific questions I was asked, outside of the obvious!
I'm not certain why I didn't get in last year. My interviewer didn't have my most relevant letter of recommendation when she called me to interview, and I always wondered if that negatively impacted me, or if that letter was missing from my application the entire process. To be honest, Penn attracts some of the most well-rounded, academically impressive and interesting applicants. I felt like my application was strong last year, but I was up against some seriously impressive competition. You just never know! I was accepted into a different program last year (but opted to not go at the last minute for a variety of reasons) and so I never directly inquired about why I was denied.