The historic centre of Siena is almost entirely medieval in plan and appearance. Fortunately is has been effectively pedestrianized since the 1960s. So there is no need of individual or public transport means to explore the narrow streets and alleys. To discover the surroundings of the city such as "San Galgano" or "Monte Oliveto Maggiore" a car is very useful. There are municipal buses running towards San Gimignano, Montalcino or Pienza, but not toward the first named.

You will find car rentals (Hertz and Intercar) in via San Marco, prices for a Fiat Panda e.g. can climb to 100 Euro per day. They also rent Vespas.

So, just walk around Siena. It's really neat with it's large stone streets. A car could fit, but i highly recommend walking. It's not like there are parking spots.