for what it's worth, I got a tourist/travel book (CADOGANguides Northern Spain) to research this a bit...everyone has different tastes, but you might want to look at something like that if we don't have enought rec's. you will be walking through an area that has a LOT of tourism unrelated to the Camino
OK, spoiler alert...are you sure you don't want to just look for things yourself?
San Sebastian (Donostia)--food, beaches, food, San Telmo, food...ok, beaches can go anywhere on the list
Getaria (Guetaria)--so Magellan wasn't the first to circumnavigate the world? worth a stop for lunch and look around but not several days
Gernika (Guernica)--obvious
Bilbao--well...Bilbao...whatever you want to see...art, history, religion, food...if only I had a week here
[Near Markina (Marquina) San Miguel de Arretxinga--worth a stop to see but not a day]
Santillana del Mar--Colegiata, altamira...I think most people wouldnt mind an extra day here
Comillas--OK, disclaimer, I'm a big fan of Gaudi, Lluis Domenich i Montaner, Josep Llimona, and Modernista...I could spend an entire vacation here, and may return to do that
I tried to see the bufones in action, near La Franca and near Llanes but without success...I read fall and winter are better months...just mentioning in case your guidebook doesn't mention
Oviedo...I spent four days here all together transitioning from several Caminos...would have liked more--churches, markets, art (take a selfie next to woody allen) Camera Santa, spend an evening listening to the Las Pelayas (nuns) sing, or check out a rock band...
Lugo--art, religion, and architecture all rolled into one in the cathedral; museum, a bit of Celtic and Roman history, the only city (I think) with it's walls completely intact (good for a morning stroll on top with the locals)
[see Santa Eulalia de Boveda while walking on the Primitivo]
edit: on re-reading, do you mean you only want to take rest/explore days in small towns...then beaches. everywhere. I liked my hotel in Playa de Borro and spent several nights here while walking ahead and taking the train back. nothing to see but the beach, but you can listen to the waves at night, and see the beach from your balcony...five steps to the beach