5 days from Biarritz to Bilbao

From Biarritz to Bilbao it´s just a bit less than 100miles, so I would recommend to visit Hondarribia and Pasai Donibane, then Getaria (Asturias is another region, not in the Basque Country, 3h drive), Zumaia and the flysch, Lekeitio, Ea, Elantxobe, Bermeo and then Bilbao.

There's a logistic issue to have in mind: it is not economical to rent a car in Biarritz and return it in Bilbao. And you'll certainly want a car for your day trips.

Two options:
1. Easiest: Uber from Biarritz to San Sebastian airport (80-100€), and rent a car there. Not cheap but much cheaper than an int'l rental car drop off fee from France to Spain.
2. Cheapest: Bus or train from Biarritz to San Sebastian (I suggest the bus, faster and more convenient), rent a car in central San Sebastian. Visit San Sebastian while you're there; you can leave luggage at the bus station before taking your car if needed.

In both cases, return the car in Bilbao.

For your base, it would not make sense to go back to France, and even Hondarribia is too far east to my liking. Getaria, Zarautz, Zumaia would suit better IMO.
And I would consider spending the last night or two in Bilbao itself.

Thanks for the info.
If we are flying back to the states out of Bilbao and want to stay in Getaria and San Sebastián 2 nights each does this make sense and is Getaria or San Sebastián closer to Bilbao ? What I’m asking should we go to San Sebastián or Getaria first after leaving from Biarritz?
Thanks

If we are flying back to the states out of Bilbao and want to stay in Getaria and San Sebastián 2 nights each does this make sense and is Getaria or San Sebastián closer to Bilbao ? What I’m asking should we go to San Sebastián or Getaria first after leaving from Biarritz?
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Getaria is very small, so unless you plan to visit wineries there and have lunch at any of its famed restaurants, two nights seem too much. It´s a just 20min drive from Donostia-San Sebastian. Getaria is closer to Bilbao, 45min drive. From Biarritz (not that much to see there) you should go to either Hondarribia or Donostia-San Sebastian first. Getaria should be a half or full day visit (Balenciaga Dress Museum may be a must), it´s also the hometown of the first person that circumnnavegated the world (no, not Magellan!). Then you can drive towards Gernika, Bermeo...and then Bilbao, where I would spend at least two nights (if you have not been before, it´s a beautiful city with a great old town, architecture and museums, not just the Guggenheim).

I fully agree with Mikel. Getaria and Donostia/San Sebastián are very close to each other, you don't need to stay in both. Pick one place and do day trips.
And likewise re. Bilbao, great city, definitely worth a full day.

Another vote here for Hondaribbia and Bilbao. Without knowledge of your tour, with five days, I would: spend one visiting the French Basque villages of Sare, Espellette and Ainhoa; then a day in Hondaribbia if only to eat at Bar Gran Sol; a stop at Chillida-Leku en route to San Sebastian, there mostly to see El Peine del Viento; and two days in Bilbao with, possibly, a side trip to Gernika. Yes, a lot of moving around but so much to experience, all so close together and not so much density/intensity in any one place.