No worries about the random questions. I can't promise anything, but I never had anyone ask me to sketch the structure of uracil or anything. A lot of PIs just want to answer your questions, talk about their research as well as yours, and generally get a feel for who you are. Some school weekends hold the interviews on the first day and others hold the interviews on the second day of your visit.
When is your interview? Grad students help out throughout the 2 weekends, so I'm sure we'll meet each other at some point. There's also a grad student only event on the second night so that you can meet everyone else in the program.
adizzygirl, I felt like an average applicant too, still do actually. Not to mention, most of my research background is in marine biology, not neurosci. But a lot of programs pride themselves in accepting students with a wide range of backgrounds. So you're one step ahead on that point.