Best Daytrips from Rome?

My first thought is Orvieto, Hill town about 75 minutes north of Rome on the regional train; under 16 euros round-trip. The less-frequent inter-city trains shave off just a few minutes, and I assume they would cost quite a bit more if you bought the ticket at the last minute. At the end of the train ride you cross the street and take the funicular up to the old town.

Orvieto's popular with day-trippers, so I'd go on a weekday rather than a weekend. If you can get there early or stay into the evening, it will be less hectic. There are quite a lot of sights (fab cathedral), so check ahead of time to be sure you won't hit days when interesting ones are closed. But it's the sort of old/picturesque town I love just wandering around. It's definitely worth a full day; on a longer trip I'd recommend 2 nights.

Tarquinia and the Etruscan tomba.
Really wonderful.

Orvieto. Definitely. It's actually worth staying there a couple of nights, but makes a great day trip.

You might also consider doing this day trip/cooking class at a tiny hilltop town outside of Rome, called Toffia. We did this last year and it was a highlight. http://www.conviviorome.com/one-day/

I do not really consider Ostica Antica a day trip, you take the Metro and can have the place visited easily in a Morning or afternoon, bit like calling the Vatican a day trip.

However, I would second (or third) Orvieto, get there in the morning, wander and see the sights, have a good lunch and get back to Rome late afternoon.

Any more, Naples and Pompeii are a long day trip, but easily done. Train travel can be as little as an hour and 15 minutes (Orvieto is about the same), but most routes about 2 hours. Pompeii, the Arch. Museum and Pizza will fill the day, but worth it.

Orvieto would be my first choice.

The walled city of Viterbo is a little over 1.5 hours away by train.

Other hill towns are Frascati, Tivoli, Castel Gandolfo and Bolsena.

Thanks for all the suggestions so far! I really like the idea of Orvieto.

I hadn't really thought of taking a cooking class while we were there, but that might be fun as well.

Tivoli and the gardens of Villa d'Este. Simply fabulous. Take the train from Stazione Tiburtina. The Villa is about a 15-minute walk from the station. Have lunch at one of the trattorie in the pedestian zone.

Viterbo and Tuscania are also very nice.
There is a bus going there from Rome and landscape is wonderful.
Again go to Tarquinia, wonderful landscape and the painted tombs from the Etruscan time is a must.
It's a real pity no-one seems to know such wonders.

The above are good if you like the small towns. If you want to go see a different region and like bigger cities (as I do) Florence and Naples are easily doable by train if you get up early and leave late. I found that a day is plenty for the main sites in either of those. We actually we're done with Florence by about 6:00 PM and found ourselves a bit board waiting for our 10:00 PM train back to Rome.

Thanks everyone!

bhak1, How long is the train from Rome to Florence?

Train Rome to Florence (Firenze on Italian train schedules) is about 90 minutes.