Civitavecchia has not much to offer, there are shops in the center, and a nice promenade. But if you are looking for something really interesting, you might consider The Etruscan Necropolis of Monterozzi, located on a hill east of Tarquinia, which ollienbertsmum has already recommended to you. And, The Etruscan Necropolis of Banditaccia, located on a hill north of Cerveteri and is undoubtedly one of the most interesting archaeological sites to visit and has been declared a World Heritage Site by UNESCO.
The Necropolis of Banditaccia is by far the most extensive ancient necropolis of the entire Mediterranean area. Very beautiful is also the National Archaeological Museum of Cerveteri, with authentic masterpieces of Etruscan art.