Northern spain......where would be the best base to travel by car?
 

I'm helping my aunt and uncle out who are in their early 60's and very athletic and adventourous people. They would like to fly into Barcelona and stay for about 3 or 4 days and then drive up north towards San Sebastian and then onto france after 5 days of sightseeing spain. I'm not sure what town would be best to use as a base to travel norther spain. Also, would it make more sense to drive or take a train and then rent a car from that location?

Their interest are food, sightseeing markets, just emerging themselves into the culture type activities.....no museums!! The total time in Spain can range from 8 to 10 days.

Are you thinking about returning the car back to Spain, or dropping it off in France? if the later have you seen the repatriation fees for the car?
I would also advise you do not use a car when stay in the likes of San Seb. So maybe you only need a car for one or two days.

Would San sebastian not work as a base? I was thinking they will take a train to cross the border and them rent in France. To avoid extra costs.

Cars in cities are simply a waste of time and money. You do not need to use a car when based in a city, parking fees are sky high plus you are still paying for the rental. That is why I said i do not advise you use a car when in SS.
If you stay out of the city and visit mainly the rural areas then it is a different matter. I say go a head and hire one.
If they take the train to France, hire a car in France, for return in France will cut out the repatriation fee.

Do they fly back out of Barcelona? What do they want to see in France? When are they traveling? The month they are traveling will make a difference on what passes and parks are open in the Pyrenees. The autoroutes are open year round.

They could do a loop over to San Sebastian then up into France and back over to Barcelona. A fair amount of driving for 5 days.

Or, they could base in San Sebastian and focus on the Atlantic coast, taking in Bilbao, San Sebastian, St Jean de Luz, Biarrritz, Bayonne i.e. the Basque country. These coast town are very close together and easily reached by bus or train. From San Sebastian, for example, buses also head inland to Guernica.

They won't need a car in San Sebastian, in fact it would just sit in an expensive garage per reberasacra. I think it's a wonderful town and good base to explore Basque country by train or bus.

If they want to drive a loop to take in more, Pau or Toulouse, for example, they can't really have much time to establish a base.

With five days, I would focus on the Basque area with its local markets, some excellent restaurants, bars, vineyard tours, beaches, etc.

Ribeirasacra, Sorry I mangled your name!

Absolutely San Sebastian is the best place to stay as a base in the Basque country. It's fantastic.

Would it make sense to drive from barcelona then drop off the car and use trains to see the area?

Only 5 days in Spain? Or are they in the city for more days and plan 5 days exploring more of Spain?
I would not even think about going very far from Barcelona.
I mangle lots of things!

3 days in barcelona and 5 days exploring.