Should we add Siena to our 3 week itinerary in Italy?

Why not Siena? It's easy to get to by bus or train from either Rome or Florence. I guess it really depends what your looking for and where your coming from as far as distance, 2 nights would be good.

I love Siena, it's a perfect contrast to Firenze in some ways. Best to spend a couple of nights, it's especially charming early in the morning and in the evening.

Siena is an easy day trip from Florence. Take the bus, not the train, from Florence to Siena.

Kent, did you do a day trip or spend the night?

I also love Siena and spent 2 nights there. Piazza Del Campo is great for hanging out and people watching. Tuscan food is my favorite, too!

Another lovely town is Orvieto, smaller than Siena, but very quaint and a gorgeous Duomo. It can be done as day trip (closer to Rome) or spend a couple of nights. It all depends on what YOU like and how much time you are spending in other cities.

Siena is a fantastic place. Full Stop.
Don't rely on your friends, and don't schedule the entire trip before you arrive. One big mistake is to miss the little known places in Italy. Cremona, Sarzana, Finale Ligure, Bolzano, Martina Franca, Sulmona, Ravello, Todi, and a zillion more.

Richard,
We did a day trip, a full day, took an early bus from the Florence bus station to the Siena bus station.
Some travelers sleep over in Siena, but I try to avoid one night hotel stays, so it was easier to make a full day of it but to return to our Florence hotel in the evening.

The museums around the duomo are some of my favorite in Italy. For me that is 2 half days. Sitting every evening on the campo is a must but I don't recommend eating there. RS really does have some very good recommendations for food. It's a great city to walk around but remember to get away from the businesses and see the residential areas for the non-tourist Siena that is very special. 2 nights? I bet you'll wish you stayed a third!

As Jo said, why not Siena? I might add, with 3 weeks to burn in Italy, why not Tuscany? Would recommend the bus from Florence for night one in Siena. We had driven into the area from Cinque Terre, passing through Volterra and San Gimignano, and on to Siena for 2 nights. Parking WAS an issue in Siena. But the car was magical in Tuscany. So, take the "rapida" bus from the auto-stazione in Florence (next to the Santa Maria Novella train station) to Piazza Gramsci in Siena. Enjoy Siena that night. Rent a car for your second day in Siena. West of Siena, you can visit the aforementioned San Gimignano and Volterra, or head south to the Crete Senesi, visiting Montalcino, Pienza, Montepulciano, or even further south to Cortona, returning late afternoon to drop your car off in Siena. Enjoy your last night in Siena, taking the bus back to Florence the next day. All these Tuscan towns are available by bus, but you can visit many more by car, plus having the flexibility to linger, savor, enjoy the moment at a place of your choosing. Worth every penny.

We stayed in Siena for 5 nights a few summers back and loved it! It was our second visit. The first time we only spent two nights and wished we had stayed longer so we decided to return.
It's a walkable city, has oodles of charm, and has much to see. It's easy to visit some of the more interesting hill towns in the area from Siena - San Gimigniano, Montalpuciano, Montalcino, Pienza - all worth spending time in the area for.
The two times we have visited Siena we have stayed at Alma Domus Hotel - it's run by a religious order of nuns, is very clean, and reasonably priced. Ask for a room with a view...the views from this place over the city are simply stunning!

Thanks so much to everyone for all of your great input! We have added Siena to our trip.

30 years ago my husband and I traveled to Italy for the first time and rented an apartment about 10 km outside Siena in order to tour Tuscany. We went into Siena that first night for dinner since the little hamlet we were staying in had no open grocery stores by the time we checked in on Sunday afternoon. We fell in love with the place and have visited half a dozen times since -- usually as a day trip from Florence and several times for a weekend.

The Siena Cathedral is just dazzling. I had never seen anything like it (and had seen many northern European Cathedrals) We have always seemed to stumble along minor festivities associated with the Palio which is fun (we would never go during the Palio but I have a dear friend who did it with her family twice and loved it). The Torre Mangia is a wonderful medieval tower -- you can see my last climb here:
https://janettravels.wordpress.com/category/siena/
The town hall includes a medieval mural of 'good and bad government' that despite being damaged by time and salt is pretty impressive. The Campo is the prettiest town square in Italy (or arguably so) Siena would always be on my itinerary for Italy. (and if you are driving - the historic center is ringed by parking garages -- just be sure you note your route -- street names etc as the place is vaguely round and it would be easy to get disoriented and forget exactly where your garage is)