We are visiting friends in Llanes, Spain, then plan to go to tour Portugal, starting in Porto. It appears that renting an auto is the only option. Is this correct? Is it costly to rent an auto in Spain, then drop off in Porto in one day? Thanks for your suggestion.
You could drive or take the bus from Llanes to Oviedo. Catch a flight from Oviedo to Porto. The flight will have a stopover in Madrid. Iberia Airlines is one option. Here's a helpful site when looking at travel options: https://www.rome2rio.com/
Dropping a rental car in a different country typically comes with a very high fee attached. There is no quick way to get to Porto from Llanes. I don't suppose you could conjure up an interest in seeing Leon, Santiago de Compostela, etc.? The previously-mentioned Oviedo is interesting, too.