My suggestion would be to ask everything that you won't find in our brochures or on our website. It's nice to know things like student/faculty ratio and all of those stats, but your time on campus is a great opportunity to really find out if Butler is right for you. Talk to some students, ask your tour guide questions about their experience here, look around campus after the tour and just get a good feel of the place.
And if you have any questions after that, hop back on here and ask them! :)