I am planning a 10 day trip with four nights in Barcelona and 4 nights in Madrid and was thinking of going to San Sebastian for two nights. Is that a bad idea? Is San Sebatian too far out of the way requiring long days of travel? Is San Sebastian even that great? Would I be better off going to places around Madrid like Toledo, Salamanca, etc?
We enjoyed our stay in San Sebastian, it is a long drive from Barcelona and Madrid though. San Sebastian has lots to see and is a lot bigger city than we had expected. St Jean-de-Luz (just across the border in France) is more of a seaside town. We would spend the extra time in Toledo, Segovia, Avila, Salamanca, etc.
I pretty much grew up on the north coast (Bilbao) and was never that impressed with San Sebastian. Been back along there a couple of times in recent years and remain unimpressed. With the time you have, and if you're driving, probably the most you'd want to detour off a direct line is Pamplona (interesting) or Zaragosa (amazing) (Zaragoza is along the way by freeway, probably). I'd save my time for the western loop that Bob mentioned, which is why I suggested weighting Madrid over Barcelona in your other thread.