San Sebastion in Early Sep.

It was fun to try the pintxos in both cities. I would ask your inn for suggestions of places to try. San Sebastian has an amazing Basque museum. Highly recommended to learn the history.

Multiple fantastic pintxo bars on Fermin Calbeton Kalea in the old town.

Walk the promenade on La Concha Bay.

The beach next door at Zurriola often has waves.

Take the bus to visit Hondarribia which has three castles.

Have fun in Donostia/San Sebastian!

We love San Sebastián! Most of the time when we are there we eat are way through town. Lol! There is so much good food in that city. One time, we took an evening food tour with Secret Food Tours. It was a great way to taste through a variety of neighborhoods other than the popular Centro (zentroa). We loved the pinxos in the Gros neighborhood. Lots of fun places filled with locals.
If you are in to soccer/football it is fun to see a game. Hike up to the Monumento del Sagrado Corazón. Lovely hike with beautiful views. It still may be beach weather and the beaches are lovely there. You can rent SUPs, etc. to paddle around the bay. There is an amusemant park in Igeldo (I think that's the neighborhood) which is at the top of hill (there's a funicular that takes you to the top.)
In the evenings, it's enjoyable to walk along the promenade or even through the old town.
It's a lovely city. I hope you enjoy your visit!

I second the suggestion of Hondarribia. We had a car and stopped there on the way to San Sebastian. I wasn't aware there was a bus. Highly recommend the castle area and they have a nice beach.

El Peine del Viento
Chillida Leku
Bar Gran Sol - Hondaribbia

Thankyou all for your recommendations! Any suggestions for resturaunts in Bilboa?

................................... or bars?

I really liked the untouristy interior capital of Vitoria-Gasteiz.

In Bilbao, head for the old town and Plaza Nueva - bars and restaurants galore.
If you can, visit Guernika, site of a civil war atrocity with a very moving museum.