This coming September we'll be doing a bicycle tour starting in Bilbao and ending in Santiago de Compostela. My wife and I will have about a week before the bike tour starts. We know we want to spend a couple days in Bilbao and a couple in San Sebastian. Any other close by cities worth a visit for a couple days? We'll be depending on public transportation. Thanks!
Transport a bit easier along the coast, lots if cute towns. Check out the hike along the coast from Zumaia, with some impressive rock formations.
A long tour, in hilly and rainy terrain (well, September is normally a drier month than the rest of the year). Public transportation is excellent, cheap, very safe and very clean, and covers most areas. Near Bilbao I recommend to visit Getxo (Puente Bizkaia and old fishing port of Algorta) and Plentzia, both easily accessible on the subway (buy a BARIK card, contactless, for a 50% discount on all public transport, one card is good for both). Further away, Bermeo and Gernika are worth a visit. Near Donostia-San Sebastian, my recommendation to the west is Getaria (home of the first person that circumnavegated the world...and no, it was not Magellan...; and to Balenciaga Dress Museum), for a very quaint town with fantastic seafood restaurants. To the east, Pasai Donibane (aka in Spanish as Pasajes San Juan) and Hondarribia are a must. Very easy on public bus.
Mikel...thank you so much...that's great info!