To end our holiday in Rome we (dad + 3 kids) decided to have three days by the sea, in Ostia. On reflection, we could have spent the whole time at Stella's, so easy and cheap are the train links to the city centre.
A three minute walk to the beach; one minute to supermarket; 5 minutes to the train station and, in the opposite direction, the town centre.
The apartment has everything you need and is very clean.
Would strongly recommend for anyone visiting Rome or Ostia who is looking for a chilled and quiet base within easy reach of beach and city.