Ostia Antica is Rome's beach so to say. It is extremely popular with locals and tourists as it is included in Rome's transport network; therefore a €1.50 ticket will get you to the beach. Be warned though that it is extremely busy on weekends and not the nicest of beaches. The sand is brown / black due to the high metal content (which means it gets extremely hot in the sun) and makes the sea a sludgy brown colour. I would not recommend going there!