sienna day trip

Fa, it would help to know where you're coming from so we can gauge whether it's worth visiting as a day trip or not.

i'll planning to make a day trip from Florence...

" i'll planning to make a day trip from Florence..." " will i be able to visit Sienna in a day trip (i'll stay the night there)? " Which is it? Will you stay overnight in Siena? How many days will you be in Italy? This will help to get more focused suggestions.

Fa, The bus is abetter option Florence to Siena http://www.sitabus.it/sita-toscana/Firenze-Siena2010.pdf The Cannon d'Oro is nice hotel in the center http://www.cannondoro.com/index_i.htm The alma Domus is a good budget hotel http://www.hotelalmadomus.it/inglese/paginaprincipale.php See the campo where the palio is run. Palazzo Pubblico, town hall, has Sienese art including Ambrogio Lorenzetti's Good and bad Government.
Climb the Torre di Mangia for a breath taking view of the city. The duomo is has a great exterior and is filled with works by Donatello, Bernini, Michelangelo, and Pisano as well as containing the Libreria Piccolomini. Even the floor is covered in art. The bapistry and crypt are beneath the Duomo. Spedale di Santa Marie delle scalla is a 1000 year old hospital that has huge works of art. The archaeological museum under the hospital. Museo dell'Opera del Duomo. The above is only part of what you can see in two of the piazza. There is much more. Budget hotels include Alma Domus and Cannon d'Oro. Both inside the walls. Siena is a medieval town that can be enjoyed by strolling in almost any part. After the daytrippers have left it can be much more enjoyable

Henry has some great suggestions for you. In terms of weather, October is typically changeable: it will rain at some point, so take an umbrella and a light raincoat. You may also get sunshine, and it'll be temperate.