Hop on/hop off bus in Barcelona is great, although comparatively expensive. Only problem is that one day really isn't enough time to do more than just sample this beautiful city. Cathedral was one of the highlights of our trip.
What can I say. I have a friend who says that if there is one place in the world that everyone should visit, it's Rome. I think she is right. We stayed four days at the end of the cruise, and still couldn't do it all. St. Peter's was the highlight (and we're not Catholic).