Two weeks Spain in September

Hola.
A 5/6 day Camino can be very rewarding, and combining it with a week at a "sunny beach spot" sounds wonderful. As with all short trips, logistics can be a key consideration in your planning. For example, for my Camino (two weeks duration, 10 walking days, one rest day) involved flying into CDG, then regional jet to Biarritiz and shuttle to SJPP; flying into MAD and getting to SJPP was considerably more complicated and time consuming (note: we did fly back to the US from MAD).
SJPP to Pamploma was spectacular and for hearty walkers a doable 3 day segment (could be expanded to 4 days for a more leisurely pace). Perhaps, you'll want to stay an extra day or two in Pamploma before walking on to La Puente Reina, Lorca, and Ventosa.
Once you've finished your Camino, you may want stay in nearby San Sebastian (it's on the Golfo de Vizcaya) for your beach week.
Other considerations - back packing and staying in refugios versus a self-guided tour staying at B&Bs and luggage transfer.
PM me if you need more info.
Buen Camino.
Craig

This is most helpful. Thanks Craig!
My only apprehension with spending the second leg of the trip in San Sebastián is that my understanding of the weather on September to be considerably cooler than on mid-summer. Thoughts?

Even in mid-summer the weather in the Basque Country cannot be counted on for beach weather 100% of the time; it's actually a great place to go to get away from the oppressive heat elsewhere in Spain. If you're looking for reliable sunshine, I think you need to cast your net a little wider. I am not a beach person myself, but I believe September should be OK on the beaches north and south of Barcelona. To me, a major advantage of this area is the variety of sights accessible for day-trips. (Again, not a beach person, so I'm all about the surrounding sightseeing.) You have Dali sights in Figueres and near Cadaques, the beautiful Girona, Montserrat, Sitges, Barcelona itself and more. I recommend staying in Barcelona where you'll have access to many great day-trip opportunities if you can tear yourself away from that wonderful city. Girona is also worth a couple of nights.

Tarragona would be an interesting stop between Pamplona and Barcelona.

I found Pamplona "OK", not exceptional. If I had a couple of extra days, I wouldn't want to spend both of them in Pamplona; given all the other options, I thought a half-day was about right and would have been stir-crazy after a day, but YMMV. Pamplona does offer a good opportunity to commune with other pilgrims.

Most of the pilgrimage stops are quite small towns, so they're also probably not the place to spend extra days.

I really enjoyed my day in Vitoria-Gasteiz, the capital of the interior Basque province of Alava. The large historic district is up on a hill. Very atmospheric. As of last year one could take an interesting English-language tour of the cathedral, which is closed for restoration. The city also has several museums. Note that Vitoria-Gasteiz appears to be in the Basque Country rain belt.

Accessible by bus from V-G but closer to Logrono is the very evocative small hill town of Laguardia. This is near the La Rioja wine country, and I noticed a number of wine bars. Especially if you like wine and will have a car, this could be a fun stop. Otherwise, it's probably not the best use of your limited time.

You'll be able to get very reasonable train tickets back to Madrid from whichever beach area you choose as long as you pin down that bit of your itinerary early and can commit to non-refundable/non-changeable tickets. Last-minute purchase of AVE tickets comes at quite a price.

This is a great help and it gives me some direction for further research. Thanks again!