We are in Siena for 5 days. My wife, myself and 2 daughters. Kids want to see the leaning tower. We will have a car.
One option is to drive up, and park in a pay and park just outside the restricted zone in Pisa and walk to the sights
Second is to drive to train station in Empoli and train it to and from Pisa. Drive to or back from Empoli can be via the Via Chiantigiana Road. Perhaps lunch or Dinner in Greve in Chianti
Third is to do train only....may be the least hassle, but seems a shame not to drive through he Tuscan countryside