I am fortunate to live near to Sperlonga, slightly inland near the historic town of Itri an old staging post on the Ancient Roman Via Appia. In my opinion Sperlonga deserves far more than just a day trip, as indeed does this whole area of South Lazio which is a delight to explore, and still remains unspoiled and little frequented by foreign tourists. Sperlonga is a delight to visit out of peak season, i.e. not in August, if you don't want to feel like a grilled sardine on the beach! The first time we came was in September and we found the beaches virtually deserted. Best times to visit are between April to July and September to October. But don't just stick to Sperlonga, venture further along the coast to Gaeta and Formia and inland to discover the old medieval town of Itri and Campodimele, an ancient village famed for its Longevity.
More interesting info in English about Sperlonga and Itri and other local resorts and towns:
sperlonga.shapcott-family.com