How can I get to the towns of Pienza and Pitigliano by public transport from Florence? How long will each trip take? Also any suggestions for interesting things to see and good local restaurants?
Pienza is 75 min from Siena by bus. Since Siena is 75 min from Florence by bus, if all planets align, you could get there in about 3 hours. Whether you'll be able to make it back to Florence the same day, I don't know. For sure you won't be able to stay there that long.
To make it to Pitigliano is even more challenging. First you need to go to Grosseto, then take bus. That's not doable there and back on the same day.
We always rent a car for traveling around Tuscany, so I can't help with transportation. I can tell you, however, that both towns are delightful. In Pienza, we got a guided tour of the Palazzo Piccolomini. I think I'm remembering correctly that we just went to the TI office and were able to hook up with a guide pretty quickly. There's also a diocesan museum in the church of S. Francesco. The town, itself , is an example of Renaissance urban design; and, so, worth ambling around. Until WWII, Pitigliano was known as "Little Jerusalem" becuase of its relatively large Jewish population. You can visit the remains of the synagogue and other Jewish ritual buildings. The Palazzo Orsini has a collection of Etruscan finds.
However you get to pittigliano, if you like to walk and enjoy the outdoors, i would make time to visit the vie cave in the area http://en.wikipedia.org/wiki/Vie_Cave . We stayed at an agriturismo near pittigliano and spent 2 days exploring a number of the routes, it seems to me you can do both loops and there and back again routes. Like nothing i have ever experienced before.