Day Trip to Pompeii and/or Amalfi from Rome??

It is easy to visit Pompeii. You take a high-speed train to Napoli Centrale (1h10), then an EVA (formerly Circumvesuviana) local suburban train to Pompeii Scavi (27min) and you are already right at the entrance of the archaeological site. That is perfectly doable as a day-trip, and you can stop for a few hours in Napoli before catching a train back to Roma Termini around 19.00 or so. The logistics work fine.

However, it is difficult to pack Positano "for a few hours" on a day-trip like this. Distances are short, but taking the train between Pompeii to Salerno (from other station not that close to the entrance of the archaeological park), and then a ferry to Positano or Amalfi, will take a long-ish time. Then, you are on the clock to race back to Roma Termini, because, for some stupid reason, there are no early night departures of high-speed trains from Salerno to Roma Termini. Summing it up: Pompeii is less than 2h away from Roma Termini (perfectly doable), but Positano is further away and requires you to gamble on arriving on time for the last high-speed train of the day from Salerno to Roma Termini.

Ostia Antica and Pompeii are different experiences altogether. I'd not exactly say one replaces the other. It depends on your tastes and interest.

So stick to Pompeii and enjoy Napoli. Alternatively, and much more easily, you can take the same suburban train you used to reach Pompeii to arrive at Sorrento, spend some 3h there, and then catch a fast ferry (aliscafo) to Napoli.

Yeah, I would not take a guided tour but I would recommend the self tour day trip of Pompeii via the train.
Would not recommend doing the Amalfi Coast as a day trip, self or guided tour ; is too far.

I wouldn't recommend a day trip to the AC from Rome either. It's just too far. I even have issues with doing Pompeii from there; Ostia Antica is so much easier.

I'd save the whole Naples/Pompeii/Amalfi Coast for a separate trip when you can give it some time. These day tours would involve WAY too much time on a bus for us. The best way to enjoy this region - as with the Cinque Terre - is during the hours when the mob of daytrippers aren't around.

Do Ostia Antica. You won't be disappointed.

Have been to both Pompeii and Ostia, on different trips to Italy. Each is unique, but each gives a "look" at how Romans lived and worked. Thoroughly enjoyed each I would suggest going to Ostia, rather than a marathon day of going to/from Pompeii in one day from Rome.
Take time to enjoy your trip and leave something as a reason to go back

I did Pompeii from Rome in 2015 - just couldn't get that close and not go - did use City Wonders for a marvelous tour.
Used the high speed train and was back in Rome before 2:00 - it was marvelous. Hope to return one day and spend several
more hours. Go For It!

Julibelle, your question is asked about twice a month, so the Search box may show you a lot more details, in past discussions. It's hard to tell from this thread so far, but a key point is that for people who have the time and location (for example, those sleeping in Naples or Sorrento), Pompeii is incredibly easy and cheap to do INDEPENDENTLY. It's still doable from Rome, even if it is a poor use of your time in a major city of the world, loaded with history and art. But once you involve a tour provider, you are adding a layer of cost and regimentation.

BTW, anyone who got back to Rome by 2PM did not spend the 2-4 hours that is the absolute MINIMUM for a real visit to the massive site of Ancient Pompeii. Many posters here say they spent the entire day. We spent 4 or 5 hours even though it was drizzling, from our 5-nights in Sorrento. That's one reason we stayed in Sorrento.