Transportation - Porto, Portugal to San Sebastian, Spain

Is renting a car an option? That would be a great way to travel. Some interesting places to visit between the two cities in northern Spain. Santiago de Compostela, Comillas, and Bilbao to name a few...

What do you mean by "convenient and efficient"? This is practically all the way across the Iberian peninsular. Rail links between Spain and Portugal are currently very limited although it might be worth looking into the Surexpo overnight train which links Lisbon with north-eastern Spain and the French frontier. Not sure about buses but check on www.eurolines.com which covers most long-distance international buses in Europe. But in general this is a trip to do by air unless you're planning to make it a grand tour of Spain and Portugal with lots of stops to sightsee on the way.

I think that you find that renting a car is very expensive due to the international drop charge. A less expensive alternative is the bus. You can get the schedule here: http://www.alsa.es/portal/site/Alsa

Click on the "Routes and Schedules" tab.

One option is to take public transportation from Porto into Spain and check the price for picking up the rental in Santiago or Vigo for example and dropping off in San Sebastian. I will say that road tolls and parking fees in northern Spain can be expensive.

Also check www.whichbudget.com to see if there are any discount airlines that fly from Porto to near where you want to be in Spain.