Lavinio, Anzio, Nettuno

I think time of year may be a determining factor.
This should help: https://www.visitlazio.com/en/
I’ve rarely been disappointed in a seaside visit, but why did you choose it?

Nettuno has a tiny but very picturesque seafront medieval walled town core. Anzio is a handsome harbor town, with an abundance of good seafood restaurants, the ruins of Nero's beach villa and a museum on the WWII Allied landing. Lavinio is a pleasant if unremarkable beach resort town, also with some good restaurants; the Tor Caldara reserve next to it is nice for a short stroll in thick forest with volcanic sulfur springs and a medieval tower on a small cliff. All have quite decent sandy beaches, with both free and beach-club stretches.

Worth it? It depends on your interests and time. Rome can keep you busy for months, but this can be a nice change of scenery, and the beach is nicer than Ostia (and Fregene too, I'd say).

If you have time, in the summer there are hydrofoil ferries from Anzio to Ponza, which is a gorgeously scenic island; the train to Anzio and hydrofoil is the easiest way to get to the island, I'd say, and you may as well check out Anzio along the way.

Edit: random shout out, but the huge delicious donuts of the Apollo 13 bar in Lavinio are by themselves a rather good reason to stop in Lavinio, if you're driving. By train Lavinio is inconveniently far from the station.

Hi, Giangilberto from Milan, I only know Nettuno, having visited a few years ago, but I’d say it’s well worth a visit if you have the time.
Nettuno is a medieval seaside town that, in addition to its spacious beaches, boasts a charming historic center.
It is a town characterized by picturesque alleyways, lovely squares, historic buildings, and cylindrical towers.
Nettuno has a long history, still evident today in the town's historic buildings, ranging from the Palazzo Baronale to the Palazzo Pamphjli.
There is also the Collegiate Church of San Giovanni Battista, originally medieval, it was completely rebuilt between 1738 and 1748. Nettuno is home to the Allied Landing Museum, located inside Forte Sangallo, as well as the Antiquarium, where you can view historical, artistic, and archaeological artifacts related to the city.
The seaside town also boasts a beautiful park overlooking the sea, Villa Borghese, a green space that marks the boundary between Nettuno and Anzio.
Regional trains run hourly from Roma Termini and take just over an hour to arrive; alternatively, the travel time by car is very similar depending on traffic, it takes a maximum of an hour and a half.