Santiago de Compostela to Badajoz

In Portugal, I found Viseu and Guarda worth visiting, though I think they will not be on your route of travel.

I haven't been to Badajoz, but the photos of the Alcazaba are very dramatic.

Well north of Badajoz is the fairly small town of Ciudad Rodrigo. Very untouristy except for the occasional bus group seemingly from Portugal (or perhaps Galicia). You can walk on the wall.

Moving south, the much larger city of Plasencia has an attractive historic center, but with limited time I'd bypass it and go to Caceres and Trujillo instead. Caceres has a large, quite pure medieval district. Trujillo's seemed a bit smaller but still worthwhile.

East of Badajoz is Merida, which is deemed to have the second-best Roman ruins in Spain (after Tarragona). It's another worthwhile stop.

If you are traveling during the summer, be prepared for relentless heat in Trujillo, Caceres and especially Merida. (I don't remember about Ciudad Rodrigo.)

Thank you--we're traveling mid-May. Should be great.

Yes, I think you'll live. But it was already quite hot when I was in the area between May 30 and June 3 last year. Merida seemed to be the worst spot. Someone local said it was the humidity from the river. It will help a great deal if you can get up early and do a lot of your walking around in the early morning. Alas, I am not an early riser.

Spend your first 2 nights at your chosen location on the Douro, perhaps around Lamego.
Next head to Salamanca for.a night, then on to Caceres for the last night. Visit Merida on your way in to Badajoz.

Puebla de Sanabria and its castle are nice, as is Salamanca, Zamora, and Merida with its Roman ruins are all nice. Trujillo is good for a couple hours. There aren't many small towns between the cities in that part of Spain and during the summer months it's brutely hot in extremadura. If you're lucky, maybe the cork trees will be being harvested when you're passing through.

Thank you everyone!