We are heading out to Pisa for a day (will have a car) and would like to see a coastal town.

Is Livorno a good choice? Don't want to drive too far up the coast as we want to loop back through StGimignano on our way back to Florence.

Livorno is a busy port town. While there are a few places of interest (churches, fort) my family usually goes there to eat at one of our favorite seafood restaurants in Italy, Da Gennarino. Since you want to go to San Gimignano, take the S1/E1 coastal road south and stop in a small town like Rosignano. At Cecina, take the S68 toward Volterra/San Gimignano.

That sounds like quite an ambitious day! I have been to all towns mentioned but over the coarse of a visit. I don't remember the distances--have you mapped it out?

You are short drives away from some of the best countryside in Europe (maybe the world) - Livorno is a characterful buttling port town but I personally would head for the hills (any of them!)

Forget the coast and see Volterra on your way home. Do you really plan to drive in Florence?