the Tuscan coast and seaside are really beautiful - but probably you know it
Since your reference point is Pisa, my first suggestion is the Versilia region.
Versilia is famous for its long beaches, blue sea, beautiful hotels and a lots of entertainments. There are many nice towns, such as Viareggio, that is famous for its seaside avenues.
Within 50 km from Pisa stand Forte dei Marmi,that is famous for its fashion boutiques and luxury villas. I suggest you stay at Hotel Franceschi in Forte dei Marmi, a nice 3 stars hotel in the center of the city.
If you want stay closer, Marina di Pisa is less than 15 km from Pisa. It's quiet and nice for families.
Going southern, leaving Versilia, you can go beyond Livorno and reach Castiglionecello or Quercianella. Both places are beautiful and are characterized by rocks. As a matte of fact there aren't sand beaches but rocks' beaches. They are about 60 km from Pisa.
Other nice seaside town is Marina di Cecina. There you have sand beaches and a relaxing pine woods.
Then if you don't mind going further that 100 km from Pisa I would like to suggest you two gorgeous locations in Tuscany:
Maremma and Elba Island
I'm personally in love with the Maremma area. It's the wildest part of Tuscany and has breathtaking landscapes. and is certainly a nice town where to stay. If you go to Maremma and you want to experience it truly you have to stay at a farmhouse. I suggest you stay at La Valentina Nuova farmhouse near Talamone. In Maremma you can find both sand beaches and rocks.
Then Elba Island is also really beautiful. It is characterized by blue sea and small towns to visit. You need to take the ferry, but it really worth a visit.