I will be in Bilboa July 8, 9 and 10th. I want to day trip to San Sebastian one day, visit Gaztelugatxe another and do a walking/eating tour. But I am having difficulties finding any tour companies online that allow for a single to book in with a larger group. Viator's tours seem to require at least two people. Other day trips are all private and I am not interested in spending $800+ on a day trip. I will happily join others. Does anyone have any suggestions?
Bilbao- I saw many tours to San Sebastián on Trip Advisor with the only restriction being age of children allowed. There is a bus from Bilbao’s airport to San Sebastian( and back) if you don’t require a tour. I hope you find what you are looking for from Bilbao. It is a beautiful area.
I'm sure you can find something by using the "Toursbylocals" tip, but in addition to buses from the Bilbao airport, there are buses to San Sebastian from Bilbao itself. San Sebastian is very walkable.
Self-guided
Take http://www.euskotren.eus to https://theculturetrip.com/europe/spain/articles/10-things-to-see-and-do-in-guernica-spain/ and visit at least http://www.museodelapaz.org
BILBAO
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The Euskotren trip between San Sebastian and Bilbao is very scenic, but it is much, much longer than the trip by bus.