It's hardly original but I loved Rome, there really is something for everyone. We were only there for 4 days so we had to rush things a bit more than we would have liked but I'd love to go back and spend more time just wandering.
Based on places I've been to in the last few years I'd recommend Montpellier, Udine and Valencia. I've also done a few of the usual suspects; Rome as mentioned above, Berlin and Barcelona and would recommend all of those as well.