regional trains/buses from Civitavecchia

We are taking a cruise in April, and have one day in Civitavecchia. Since we are spending a week in Rome prior to the cruise, we would rather go somewhere on a regional train or bus, e.g. Santa Severa. Are there on-line bus or train schedules departing from the Civitavecchia port so that we can try and plan a day trip that does not require us to go into Rome? Any suggestions for a day trip? Thanks.