richardg, you might check with the hotel the tour will be using; our Basque tour stayed at the Petit Palace Arana, and we loved the price, staff and location. We stayed on for several days after the tour, and would gladly stay there again.
I don't know about "must see," but for us the things we enjoyed most, and would recommend, are the Archeological Museum, and the nearby Holy Week Museum, Museo Pasos de Semana Santa. Wonderful dioramas and awesome life sized statues that are carried on floats in the Holy Week Processions.
We also enjoyed the Museo de Belles Artes, It had been suggested by some that this one was not worth seeing, because it's currently under renovation, but the exhibits were wonderfully imagined and displayed.
I posted a tour report https://community.ricksteves.com/travel-forum/tours/tour-report-basque-country-of-spain-and-france-september-22-30-2023
I tried to copy and paste just the section of the report that dealt with our post-tour activities, but it exceeds the character limit for individual posts. Skip down to "Day 9" in the tour report to what we did on our own.
This is a really good tour; have a wonderful time.