I second (or third) Ostia Antica. I LOVE Pompeii, but it requires a full day devoted to it from Rome and even then you miss out on a lot. I highly recommend the Context Travel tour
at 75 Euro per person. You get a personalized, highly knowledgeable guide and very small groups (max of 6 people). http://www.contexttravel.com/rome/to...nked-tours= yes . If it fits into your schedule try the visit ...