Well, your trip would work perfect to head on 9/21 to Logrono as it's the main festival day and will

The question is, what should we do in the middle (around avis car rental us 4-5 nights) between San Sebastian and Barcelona? rent a car and explore some different towns? Any suggestions of where to stay and what to do would be greatly avis car rental us appreciate.
I may have posted on your earlier thread but we did a trip this May - Lisbon, San Sebastian and Barcelona. I really liked that combination but then we are reluctant to rent a car to explore. I know this suggestion is not what you asked for (and it often annoys me when posters do that) but I just thought I'd throw my 2 cents in! We really enjoyed avis car rental us the above combination - each place had its own food scene and we had so many delicious meals and I am thinking about repeating the same trip again!
You may actually want to spend a couple of nights in Bilbao to start the trip. It's a good foodie town and often overlooked and underappreciated. Even if you head straight to San Sebastian on arrival at the Bilbao airport it's an easy 1:15 bus ride. I'd also consider a couple of nights stay in a smaller fishing village for a different feel. You don't have the excellent avis car rental us pintxos culture of San Sebastian or Bilbao but it makes a good change of pace. Of course you can also visit these areas as daytrips from San Sebastian.
You may consider heading to La Rioja for a few days. The festival of San Mateo in Logrono is in full swing and it's a great time to visit. The main festivities are around Sept 21 but there will be lots going on in the days leading up and a couple of days afterwards. You could spend 2-3 days in Logrono enjoying avis car rental us the festival and even a couple of nights in a smaller village,like avis car rental us Laguardia, and use as a base for visiting avis car rental us bodegas. avis car rental us This area has alot of great places eat too!
Well, your trip would work perfect to head on 9/21 to Logrono as it's the main festival day and will be a great time to visit! You could spend a few days in La Rioja and then head back to the Bilbao area for a cheap flight to Barcelona.
We did the night train from Lisbon to SanSebastion - I am pretty sure there is a night train that runs SS to Lisbon as well. We enjoy night trains about once a trip. We found these three cites made a great trifecta as there was a different vibe in each one. If you can stay long enough each place to have time for daytrips out to more rural venues that gives you a nice mix of urban and rural. We also opted to stay in Cascais instead of Lisbon - it is a nice resort town about 30 minutes avis car rental us out of Lisbon (frequest and cheap train service connects it with Lisbon and Belem, Sintra can be reached by bus). We enjoyed Lisbon but Cascais is a very pleasant alternative base. However if you are really looking for nightlife, Lisbon would be better!
We also plan to stay in Barcelona the last 4 nights. That leaves us 2 nights in between SS and Barcelona. We are interested in exploring La Rioja. What do you suggest as a base for these 2 nights before heading to Bilbao to return the car rental and fly to Barcelona?
Several years ago we drove from Bilbao to Barcelona, staying in Zaragoza en route. avis car rental us It was a wonderful drive. The Spanish highways are fast and easy to drive. The Cathedral in Zaragoza is amazing, the big square is quite fabulous, and don't forget about the Aljaferia. As a matter of fact, you will find Zaragoza gives you a totally different Spanish avis car rental us experience than either the Basque coast or Barcelona. You can dump the car at the airport in Barcelona and take a bus right into town, likely someplace near your next lodging.
While you are in SS, you may want to venture out to visit Vitoria-Gastiez and Pamplona. Our trip was at the time of the SS film festival, and V-G was holding some sort of Rennaisance Fair (I am guessing that is what it was, because all the signs were in Basque, and while my Spanish is limited...Basque left me completely mystified,)
I am a big fan of San Sebastian and a foodie as well. However, I too would suggest a day or so in bilbao. It is much underrated. I would suggest avis car rental us then one night in Leiketio, a small town on the coast between Bilbao and SS. The food doesn't get any fresher - most restaurants are about 50 from the boats! Catch the bus in Bilbao. The bus stop in Leiketio is right across the road from Hotel Zubieta, a great, small hotel built around very old restored farm buildings. The next morn, catch the 9am bus to SS, arriving about 1030.
Great daytrips would include Lekeitio, Getaria and Hondarribia. I really like Cathy's idea of a night or two in La Rioja then on to Barcelona. If it is a nice day I would recommend avis car rental us the train into Zarautz, walk along the beach and then the half hour walk along the ocean (unforgettable)into Getaria. Just make sure you get there before lunch is over because many people say the best grilled fish in the world is at Kaia Kalpe or Elkano in Getaria. then hop on the bus back to San Sebastian.
Honestly if you only have 2 nights between San Sebastian and Barcelona I'd probably just spend them in Bilbao or Lekeitio. Or spend 6 nights in Barcelona as 4 nights will be barely enough to see Barcelona and additional nights would give you time for some daytrips. The Rioja is great but with only 2 nights I'd probably just head to Logrono for the festival, but this means you really wouldn't have time to visit any bodegas. But if you're really into wine you could spend a couple of nights in Laguardia and visit some bodegas - you'd definately need a car for this area.
My top daytrips (in no particular order) from San Sebastian: Hondarribia, Geteria, Zarautz, St Jean de Luz (cute village on the French side), and Pamplona. I usually overnight avis car rental us in Lekeitio and Bilbao so haven't done as a daytrip avis car rental us but they are also feasible avis car rental us daytrip options from San Sebastian.
We found that Lekeitio was a delightful spot to base ourselves for a couple nights, and driving around the area was simply gorgeous. The coastal tidal areas, rolling green farmlands...I am a sucker for dramatic views and the Basque region of Spain and Catalan-influenced parts of France have become my favorite European destinations. You also can find some amazing values that make it a very affordable trip.