bus and train is possible to get around
You can certainly get around by bus in the immediate surroundings - and for longer day trips, you can head back to Pisa by bus and then use the train to move around to Lucca, Florence or Siena, for example.
Depends whether you want to spend most of your time at the resort and relax by the sea and just a few days of traveling around, then bus+train would be fine. If you want to head further away from the area, a car would definitely be useful to cut back on travel times. So totally depends on what you'd like to do during your holiday and how many days total you have!