Ciao! Since it seems you are driving to Lucca, then Pisa and then on to Volterra all on the same day (read on another of your posts), then I don't really recommend trying to fit other stops and sights along the way. Unless I understood incorrectly and you are doing the driving on separate days?
If that is the case, then you could stop for a brief visit to Pistoia, a small town usually off the main tourist stops. The main square has a beautiful cathedral and baptistery and the museum of sacred art has several treasures.