Vacation starts tomorrow, best option for day trip from Florence to Siena?

Siena is indeed worth a visit. As a city I find it more attractive than Florence. Less (but still enough) things to admire, but more atmosphere.
There is an hourly direct train (leaving at 10 past the hour, taking 1,5 hrs) and at some hours one with a change (leaving at 28 past; taking 15 mins longer) from the main station in Florence (Santa Maria Novella), taking 1,5 hours. The station in Siena is quite a walk uphill, so a taxi will be practical.
For the return trip, it's about the same rhythm: direct trains leaving Siena at 18 past the hour and some indirect ones leaving at 41 past. Your latest chance to get back is the 21.26.
Check at http://www.trenitalia.com/tcom-en

Sorry for the cut-and-paste error. This reads better:
"There is an hourly direct train (leaving at 10 past the hour, taking 1,5 hrs) and at some hours one with a change (leaving at 28 past; taking 15 mins longer) from the main station in Florence (Santa Maria Novella)."

Another option is taking a bus. We found it convnient , low cost and bus station to siena central an easy walk.

The easiest way to get to and from Siena is on the bus--1 hr. ride. Walk out the side door of the Florence train station--to the right of McDonalds. The buses there take you into the Siena city center.
You're right that Siena is especially beautiful after dark.

We took the bus and it was pretty convenient. Sienna, as stated above, is worth the trip.

Siena is best visited by bus, which takes less time and drops you on the city center on top of the hill. The SITA bus station is across from the Florence train station, on via Santa Caterina da Siena.

For the return trip, in case you want to stay a bit later, you might want to consider the train, which runs later than the bus. The last train departs from Siena at 9:26 pm (the last bus departs from Siena shortly after 8pm).

All I can say is thank goodness I am not the only one still making final plans for our trip and we leave in one day!!

For what it's worth we are doing a day trip to Tuscany with Walkaboutflorence, full day to Sienna and lunch at a farmhouse, two other towns visited also. Have fun!

Thank you to everyone who responded, your input is much appreciated. I'm looking forward to checking it out, and now you must forgive me.....I have to go pack!

I would also suggest using the Bus from Florence to Siena. Check the schedules and try to get a Corse Rapide (express) bus as it's a bit faster, with a travel time of just over an hour as I recall. As Roberto mentioned, you'll be dropped in the main part of town rather than the rail station, which is at the bottom of the hill. Don't forget to validate your tickets prior to boarding the Bus.

If you want to stay later in the evening, you may have to return to Florence by train, as the trains run later than the Bus. That's not a big deal, just take a Taxi down to the station. If the train you use is a Regionale, you MUST validate your tickets prior to boarding on the day of travel, or risk hefty fines.

Thank you!