I'm so jealous. I did the Florence, Rome, Venice trip when I was in high school. I totally concur with what DDDave said. Rome is magnificent. Especially the forum area and Coliseum. Hell, all of Rome is great. You can really feel the history there. Not sure if this would be your thing, but they also have an ancient system of catacombs there. Also, watch your wallet in the touristy areas. During a tour of the forum, one of my fellow tourists was pickpocketed. Fortunately he caught the girl who had pickpocketed him before she got away.
Of the cities I visited, Rome and Venice were my favorite. But, you cannot ever spend enough time innt. Also, don't forget the gelato and coffee. They're magnifico! Rome. It's huge. Piazza Navaronne is a definite must for free entertainment.
Edit
relenawolf is right. It is Piazza Navona. Oops!
