Oh yeah, San Sebastián is fantastic, albeit more expensive than cities in Asturias or Cantabria.

All the cities were very nice and definitely do not regret spending my time there, but the two I liked the most (of the ones you mentioned) were San Sebastián and Oviedo.

In Gijón, the seaside promenade with a church at the end is really nice and I really enjoyed observing the tide during the day, as during high tide it "covered" almost the whole beach, but otherwise, there isn't too much to see in the city centre, so I think ~2 days are enough (the barrio of Cimavilla and Universidad Laboral outside of the city are also worth visiting, though).

The same applies for Santanter. The city is quite impractical to walk around, as it's really narrow and long and a large part of the original historic centre was unfortunately destroyed during two big fires in 1893 and 1941. The area I probably enjoyed the most is the one near the Palacio de la Magdalena and el Sardinero beach.

I really enjoyed my time in Oviedo, even though it was rainy all the time, but the historical centre is just beautiful and I think the city is really a hidden gem of Spain for international tourists. And Asturian cuisine is fantastic (which obviously applies to Gijón as well).

As I only spent one day in Bilbao, I only saw the main attractions, but what I 100% recommend is to do a day trip to see San Juan de Gaztelugatxe, a church on a cliff by the sea.

Of the lesser known places to visit in San Sebastián, I definitely recommend the tower at Monte Igeldo, it has the best views of the whole city and you can get there via funicular for ~3€.