Soaking in Tuscany's Hot Springs!
The unique geology of Siena means that we are blessed with natural thermal baths in the area, and visiting these hot springs can be a great day-trip or long-weekend from the city center! What sounds better than relaxing in warm, mineral-rich, soothing waters here in the heart of Tuscany? The spot that is closest and easiest to get to by public transport is the Terme Antica Querciolaia in the nearby town of Rapolano Terme, here in the Province of Siena. There is a bus stop a block away from the spa complex, which is near the heart of the town. There are baths both inside and outside, with a variety of temperatures from piping hot to pleasantly tepid. If you're on a tight budget, just go for the second part of the day, which is half-price (€11 on weekdays, €12 on weekends). You can find a full list of prices at: http://www.termeaq.it/offerte-benessere/piscine-termali-terapeutiche/
Another spot in Raplano Terme which is a little bit outside of town is the Terme San Giovanni, which is very pictureque, with the outdoor pool looking out over the Tuscan landscape. It's not quite as easy to get to this place by public transport: basically, you have to get off the bus or train in the center or Rapolano Terme and then walk 20 minutes to the spa... not impossible but not super-convenient. Their pricing is very comparable to the Antica Querciolaia, and you can see their price details at: http://www.termesangiovanni.it/
If you are feeling more adventurous, a place for a weekend get-away is Bagni San Filippo, right on the edge of the Sienese province. There is a hotel in the town, but most people just enjoy the natural stream, which is free and open to the public. The water naturally has a lot of calcium, which over the years has created these amazing rock formations which look like glaciers. The Bagni San Filippo can be reached by bus, but not super-frequently, so it's best to plan to stay overnight if you want to travel there by public transport from Siena.
For info on local bus schedules, visit tiemmespa.it or check out the "Public Transport" tab on Google Maps, which is fairly dependable.