Is one day in Siena enough?

Absolutely not!  

It is a city that has a totally different soul at night than during the day.  Daytime is wonderful, but it can be a little overrun with tourists.  That's ok though.  Just enjoy the streets, walk around away from Campo or the Duomo (after you enjoy them of course) and you'll be able to experience the quiet corners of one of Tuscany's most beautiful cities.  The key is to get lost...really lost.  Pack a bottle of water, a few batteries for your camera, a couple of memory cards so you don't have to delete a single image, and off you go.

By staying in Siena for a couple of days you can enjoy her at night.  The people of Siena are out, the streets are much more quiet, the Campo gets filled with people enjoying a glass of wine under the stars, and the Palazzo and Duomo reflect the moonlight in a way that cannot be described only gazed at.  Be sure the grab a bottle of wine at the little wine shop on the Campo.  She'll open your bottle and give you plastic cups, they offer limoncello, and bottles of beer if you what you want doesn't grow on a vine.  Be sure to clean up after yourself so Siena doesn't restrict use of the Piazza and prohibit drinking a nice glass of wine.

There is so much to enjoy in Siena that one day is certainly not enough.  Spend time, stay in a small B&B or little hotel, if you go for the Palio go the week before and enjoy the build-up to the race, take an Italian class, have a Guiness at the Irish pub in Piazza Gramsci, let Siena seep into your soul and that takes more than a day trip.