We will be 9 days in Rome in December and would like to do a 1-2 day trip to either Siena, Perugia or Florence. We will be traveling with our 20-year old daughter and 24-year old son. Any recommendations?

That depends on your interests. If you want to see the "cradle of the Renaissance" with all the art that emerged, go to Florence. (I cannot return to Italy without making at least a 3-day pilgrimage there just to soak up that atmosphere)

If you want a more upbeat, visual delight, Perugia is cool, especially for young people because it is on a hill with great views of the countryside, excavations under the town, fascinating streets to wander through, lots of college students, etc.

Siena is a town that is visually appealing within its walls, also interesting to wander through, etc.

Perhaps you should also check out some of their city web sites that list festivals going on, too, to see if that piques your interest.

They couldn't be more different, so it depends what your interests are. You might also consider Orvieto, an easy day trip from Rome.

My vote is for Florence. It will be a great follow-up to your stay in Rome. Different vibe but lots and lots of world class sights. You will be visiting in December and must consider indoor options although visits to the piazzas and overlooks of Fiesole and Piazzale Michelangelo will still be options as well. Plus it is easy to get to via train.

We just came back from visiting our daughter who is studying in Perugia. We went to both Perugia & Florence, didn't get to Siena. Perugia was fascinating, an amazing hilltown, there are escalators getting one from one end to the other & the ruins. Florence has the art, mostly the Michelango sculptures.
I would pick based on your interests but also the easist train connections, if that is how you are traveling. If you go by train, make sure you activate your ticket in the "yellow box"by the track before you depart.

The places you mention are all very different (and even have different weather in December), You also don't state anything about the interests of your grown children or your interest in visiting Italian cities. You don't mention Napoli, and that would be my first choice for a 2-day excursion from Rome in December. (Although Florence can be cold and chilly, I would pick it over any hilltown, and it is an easy trip from Rome.)

If any of you have a sharp interest in antiquity, shopping, food, wine, chocolate, nightlife or frescoes, etc -- that should be given some consideration. If you are mainly interested in going to a place with "something for everybody", Florence is the richest of the places you name, but it is also the most touristed and touristy.

I find Florence quite an upbeat place around the holidays, and Perugia may be somewhat abandoned, with the students on holiday. Given that the days will be so short, Siena and Orvieto might feel gloomy.

Thank you all for the input. We clearly need to do a bit more soul-searching before we make our decision. I appreciate the feedback.

Same trip I'm considering. Thanks, all.