I would recommend Tarquinia for charm and convenience to the airport. Civitavecchia is a little closer but not that interesting. Tarquinia has important Etruscan tombs just out of town and an archaeological museum in town, as well as a nice old town. I put in driving directions from Tarquinia to Fiumicino at http://www.euroshell.com asking to go at moderate speed; it says it's just under an hour to the town of Fiumicino; the airport should be a little less.

Andrew Missouri