A small resort community 50 km (31 miles) north of Rome, Santa Marinella is a pleasant escape from the tourist madhouse that Rome can sometimes become. Don't be misled, there are tourists at this once Etruscan port, most of them there to enjoy their villas overlooking the Mediterranean. Ignore the self-righteous tans of the wealthy and look instead to preserved medieval architecture that includes the Castle of Santa Severa, which doubles as a museum and is immediately accessible from a nice little public beach. On the way here from city center, there is also the well-tended Stabilimento Balneare Perla del Tirreno, although this where paying customers lay down their towels.
If your Rome itinerary is in need of a beach day, then follow the A12 up the coast for a relaxing day trip, especially if you have a weekday to kill.