If choosing to go via Portugal, inland (the upper Douro valley, Brangança) is more off the beaten track than Porto. You might want to take a look at my trip report which covers the area north of Lisbon with a few days spent in Santiago; click on my name to find it.

And what time of the year would this be? It makes a difference. 10 days is enough time to explore some of northern Portugal and Castilla y León, and going from Porto to Madrid through the Douro and Ribera Del Duero wine regions would make for an interesting drive, but not during the hot summer months.

The coastal route is another great option, even if you wanted to visit Porto before heading north to Gijón via Leon. You can always fly from Santander to Madrid (as low as €150/person). The flight takes 1 hour.

There are so many variations on this theme.

From where will you walk the Camino?

There is one expensive problem to consider, picking up arental car in Spain and dropping it off in Portugal.

One other suggestion is not fly home from Madrid but from London. We have done this often from Santiago. Vueling flies to London from both Vigo and A Coruna.

Thanks for all these useful comments. Michael, I am following your trip report on a map, most useful.
Robert, thanks for great idea of doing a bit of both -Porto and then the coastal route. I will look at flights from Santander. Although driving through the wine regions would be good too as we will travel in early October.
We will walk the Camino from a bit after Leon.

The pictures are pretty much in sequence with the itinerary except that I do not include Santiago in my Portugal pictures.

For now I am going to give you some brief ideas of what to see and do: This will hopefully fill in some blanks.
The larger (in area) wine regions on that route:
Ribeira Sacra
Rías Baixas
Bierzo
Vinho Verde
Douro

Cities (and not on the Camino)
Lugo
Ourense
Verin
Zamora
Salamanca
Valladolid
Porto
Tui
Valença

Naturally there are 1000s of small villages along these routes which you can explaore too.
Then there are the sights like
Caurel
Las Médulas
Castillo de Urueña
Again a few hundred more.

A normal drive between the two cities is around 8 hours(with stops)
So with 10 days I would suggest you take your time and not do one nighters, as they do not work and allow you to explore the region you are visiting.

Good luck with the planning.

You will only need one base to explore the Ribeira Sacra, not need to move around.

http://maribelsguides.com/newsletter_11_09.pdf

Thank you for all the useful information. I have printed everything out and now will go to google maps and follow the routes. I love the idea of sticking in one place for a while, maybe the Ribeira Sacra. Thank you ribeirasacra for the detailed list. Thanks for mentioning Tordesillas, Cehegin, it does look lovely.