For your requirements, Camino Lebaniego from San Vicente de la Barquera to the monastery of Santo Toribio de Liebana may be a good choice:
1- It's very near Santander, you can go from Santander to San Vicente (the beggining) in just 1 hour train ride. From Potes (the town at the end) you have direct buses which will leave you again in Santander in 2 hours. From Santander you can easily go to Santiago to continue on to Finisterre
2- It's one of the most beautiful ways, mountainous, with an incredible riverside walk in one of the stages.
3-You can do it in three or four days; as its's barely 70 kms long
4- It's a full Camino, arriving to one of the holy sites in Spain (second only to Santiago de Compostela) , the Monasterio of Santo Toribio in which the biggest piece of the Holy Cross in the world is custodied.
5- In case you're interested you can obtain a Diploma, similar to Santiago's Compostela, when you arrive to the Monasterio of Santo Toribio. The system is the same: you stamp your credential every day, and when you arrive you receive your diploma. That will give your wife a better taste of what the camino is like and also will give you a second Camino completed.
6-It has few but enough (and good) lodging facilities.
7- The only con is that it's a mountainous way, with some hard inclines; but perfectly doable to anyone with a minimum preparation. And the landscape deserves the effort.
You can learn more about that beautiful way in that forum in the section other caminos in Spain or by searching "Camino Lebaniego" or "Camino a Santo Toribio de Liebana" in the web.
Hope that helps and ¡ BUEN CAMINO, PEREGRINO!