Siena Day Trip from Florence

That trip is very easy from Florence, and travel by Bus is the usual suggestion. The Bus station is close to Firenze SMN rail station, so very accessible. If you go by Bus, you'll be dropped right in the main part of town in Piazza Gramsci. If you travel by train you'll arrive at the bottom of the hill and will have to take a Taxi or Bus up to the town. If you choose one of the Corse Rapide Express Buses, the trip is just over an hour.

http://www.sitabus.it/en/florence-siena-bus/

We used tour company Walk About Florence that does a "Best of Tuscany" all day tour which takes you to Siena, San Gimignano and Pisa with a stop at an Organic Winery for lunch. We saw so much in one day and enjoyed each town. We are going back in May with another couple and they will be going on this tour.

Siena is worth a full day. The bus is indeed the most practical way to get there.
Pisa can also easily be done on your own. There is a half-hourly direct train from the main station, taking around one hour, at the most reasonable price of around EUR 17 for a return journey.

If you can, get there early. Take a half-day tour, since Siena has a lot of art and history. Then tour on your own for as long as you can, Siena is beautiful and serene late in the afternoon. If you can stay for dinner and take a late bus or train back, you will have a much richer experience.

The buses are out the side door of the train station--to the right of McDonalds Restaurant. They line up around the block behind McDonalds.

Yes we did Seinna on our own. It was simple, we just went to the bus station. Trains leave and return often

Just recently did this day trip
Siena is really easy to navigate, as a day tripper there is really one road in town. You walk that road from the Cathedral on one end to the Campo Piazza on the other end.

Those are the 2 cannot miss spots and with exploring the cathedral the walk with nice shopping in between and lunch in the Campo Piazza will occupy a good portion of the day.
Definitely no need for a tour guide not sure any value they would add but I would recommend buying tickets in advance for the Heavens Gate tour at the Siena Cathedral
This to me was well worth the cost of admission but the tickets sell out well in advance (you get to climb up into the roof area and see the cathedral from above and also look out on the town from up high. Going inside and outside back and forth as you walk around the upper area. That ticket includes access to all parts of the cathedral complex (crypt, bapistry, etc...)

The cathedral is really impressive, I thought more so than the one in Florence which is saying something.
The Campo Piazza is amazing as well. The type of place you want to linger and people watch for an hour or two.

We drove a rental car which we had and it was very easy, parked in the Santa Catherina lot which is easy off the highway and before any ZTL zones. This is a paid lot that seemed very secure to me, multi-floor we had luggage in the car. From there a short walk leads you to a quite impressive escalator system that brings you to the top of town without climbing steps.
You take a left to get the cathedral or a right to get to the Campo Pizza ; both are 5 minutes away and between them is a 10 minute walk which is lined with nice shops and heavy with pedestrian traffic.
I am sure there is more to see in Siena but as mentioned the above basic plan sees the 2 highlights and takes up the day so we didn't have time for more.

The link below tells more about the Gates of Heaven tour in Siena and is also the link to by tickets:
http://www.operaduomo.siena.it/eng/portadelcielo.htm

If you do buy tickets you have to pick a time slot and date. They will send you an email which looks like they want a bank wire transfer deposit but that is only for large groups. No deposit or prepayment is needed. Just print out the email and bring it to the ticket office when you arrive and pay there.

Train is very easy, as is bus. Train station is at the bottom of the hill and you take a series of covered escalators up to the town proper.

The Cathedral is impressive as is the central piazza. Just those two features themselves, you can see how Siena rivaled Florence. Pay for admission to climb to the top of the unfinished wall of the Cathedral expansion project. There you'll get a bird's eye view of the Cathedral and the Piazza, along with a top-down view of all of Siena and the surrounding country side.

You should know that Siena is not an even straight line walk, it's very hilly, deep high roller coaster hills that are as difficult to go down as up. So get your comfy hiking boots on and get in shape cause you will need the stamina to make it thru the town!

We arrived in Siena by car in early afternoon in late June and all of the parking lots seemed to be full. We just parked on the street but it took awhile to find a spot, and then to walk to the sights. You might want to consider timing as part of your plans if arriving by car.

Siena is a very simple bus trip from Florence. Definitely NOT a half day as you'll miss so many of the very reasons to go there if you don't stay at least 6 to 8 hours. My recent trip there I spent nearly 4 hours just seeing the Duomo complex alone. Definitely go early so you'll have a full day and be sure to buy the 131 Rapido to shorten the bus trip. There are just a few of these each day that go and return at times that would allow a meaningful visit. You definitely do not need a tour guide. Siena is a small town. Get RS guidebook and do some internet research before going and that's all you'll need.

While in the Campo Piazza last February we saw a small temporary building where a special pastry was made. It was a small ball served hot, similar in taste to a donut from Krispy Kreme. I believe its served at Carnival time. Is it possible to find that in Siena or other places any other time? We will be there next April/May.