To me, most of the small villages and towns in the Basque region, up into southern France are worth from few hours to a day each. Hondarribia is worth the better part of a day as does Bayonne France.. Zumaia and Getaria (west of San Sebastián) combined are worth a day. Not certain if San Sebastián merits 3 days. If you’re driving, the Loire valley is very nice. In addition to the chateaus such as Chambord and Chenonceau, we enjoyed the towns of Rocamadour, Blois, and Chinon. We drove along the Loire river and stopped in some smaller towns such as Vouvray. Coming from St. Jean de Luz we drove the wine route in southwestern France, near Bordeaux, before heading back to Paris. What you do really depends on what you want to see. There are plenty of beaches along the Basque coast.
I haven’t experienced it since I pick up and drop off rental cars at the same place, but others have warned of large fees for dropping rentals off in another country, so you may want to look into that.