Anzio/Nettuno southwards, Santa Marinella northway. This if whitin 1,30 h trainride to Rome. Mind that your coming off season so the more touristic places such as Terracina/Circeo empties rapidly after the August holidays and become rather boring. Sand beaches at south. Rocky at north. Farer away, Argentario penisula in tuscany and Gaeta closer to Neaples. Aprox 2/3 hours by train.
Sorry for the reply in english