Ciao Gemma! Yes, a car will definitely be useful. The coast is really nice and there are many small towns along that area. As Cristina mentioned, some of the most well-known include Castagnetto Carducci and Bolgheri as a wine region, the famous Sassicaia wine comes from there. Head a bit further south to the
Gulf of Baratti to experience an almost Caribbean-like beach - the article offers a presentation with info on how to get there.
If you have time, definitely visit Pisa as well which isn't far away, at least for half a day! 5 days will fly by, not sure if you'll get the chance to visit Florence - if you've already been, definitely concentrate on exploring the coastal area

. You can also head a little north to the Versilia and to Viareggio to see the difference with Bibbona.