Travel from Paris to French Basque for 3-4 days in August

First, book your train tickets now for a TGV train from Paris to Hendaye which is the last French city on the border with Spain. Then you can transfer to the local commuter train called “Euskotren” for the final 30-minute ride to San Sebastián/Donostia. www.SNCF-connect.com is the place to buy your TGV tix. Euskotren tickets can be bought on-site when you get there.
San Sebastián/Donostia is beautiful with delicious pinxtos ( tapas) and fine beaches at La Concha Bay and Zurriola. Because 3-4 days is not a lot of time for the Basque Country, you could do a daytrip from San Sebastián/Donostia to Bilbao or the walled medieval town with two castles in Hondaribbia, which is across the border from Hendaye.
Bilbao is 75 minutes by bus from San Sebastián/Donostia, Hondaribbia is 30 minutes away.

This past April we did a ten-night trip: five nights in Biarritz and five nights in Bilbao. We had a blast!

We based ourselves in Biarritz in an AirBnB but had a rental car and did day trips. Bayonne, San Sebastian, St. Jean Pied de Port. We loved the vibe of Biarritz: yes, it's a tourist city, but so cool to see barefoot surfers walking through town with their boards. Also, the coastline is STUNNING and made for wonderful evening strolls. The only disappointment we had is that it wasn't a area where we found traditional French foods. We mostly just bought and cooked normal groceries from E LeClerc.

I stayed in Bayonne and liked it a lot. Biarritz didn't thrill me, but it didn't help that I hit a very rainy day there. I enjoyed walking around St-Jean-de-Luz, but it's quite small. Bayonne has a nice historic center, the Basque Museum and a chocolate museum. The latter had a generous tasting spread in 2019. The chocolate was tasty; it was not €100-per-kilogram stuff.