Small towns outside Rome by train

Add Orvieto to your list

We really enjoyed Orvieto too on a recent trip.

Ostia - assume you mean Ostia Antica as distinct from Ostia Lido, the nearby beach resort? Ostia Antica is only half-an-hour by train from Rome and the archaeological site is a 10 minute walk from the station. It is an easy half day trip from Rome. The Lido is another 10 minutes or so by train from Antica but likely not much happening in December.

Assisi is 2+ hours by train and would be worth an overnight stay.

We visited Frascati in June of 2025 as a day trip from Rome, easy to get to by train. We had unwittingly scheduled this for a Sunday or a holiday or something like that, the town was almost entirely shut down. Thus our choices for sampling some of the white wines that have made the town well-known in the US were limited. The old town was still worth wandering around for couple of hours. A shop in the Piazza del Mercato served thin-sliced pork sandwiches that were delicious.

We'd visited Orvieto on an earlier trip several years ago, also easy to access by train. I was really impressed by the basilica, even though our visit was on a chilly day.
I thought a day trip was the right length of time for these places.

Of the ones you mentioned, I've only been to Ostia (Ostia Antica) and I enjoyed it. The ruins rival Pompeii - minus the shadow of Vesuvius - with no crowds and a lot more shade. The city center is pleasant and has a small but charming area where you can walk around and pop into whatever little restaurant looks good and eat with the locals. It's not touristy at all, most places we ate we were the only English speaking foreigners.

The best thing Ostia has going for it, though, is the proximity to FCO. We chose that as the last city before we flew home on our recent trip to Italy so we could wind down at the end. So rather than taking Mr. Toad's Wild Ride through the streets of Rome to get to the airport, we had an easy 15 minute taxi ride. I think it merits consideration as a first and/or last stop if you want to ease into/out of Italy.

Only downside is that if you're coming from Rome the train does go there but IIRC, you have to make TWO changes and you finally roll into town on a rickety little commuter train. It's all cool, but just be prepared.

I've been to Tivoli, always a favourite, and Bracciano. I enjoyed Bracciano town and the lake as a whole, but at the time (two years ago) the castle was closed. Time to go back!

Of the two, I'd probably pick Tivoli as there is more to see and do, and more restaurants. Tivoli has 2 main and an interesting minor site: Villa d'Este and Hadrian's Villa are the biggies. Hercules' temple is the third site included on the sightseeing pass. It was mostly interesting because it has been repurposed so many times over the centuries and it was fun to see all its various roles. It's a pretty town to wander around and the train station is easy walking to the centre. Although from Rome, a Cotral bus might be just as easy.

Edit: the two biggies are both excellent and worth seeing. It'll be a squeeze to do both justice in one day so an overnight is a good idea.