Just walked this a few months ago. Obviously, spend some time in Porto. I'd second Baiona. It's really a very nice town. March is probably too early, but there is a very nice National Park just off the coast (easy ferry ride to get there) with some nice walking trails. I only walked the coastal route and most of the other towns are nice but not exceptional (I know I'll incur the wrath of some on that comment
). They are very nice just not all that unique.
If you get to Porto early, you could go south. Lisbon is always great if you haven't been there. I also liked Coimbra and Tomar. Tomar particularly has a very nice town center and a great castle and monastery to visit. And there always is Lourdes (more for its unique nature than its architecture or town). The walk from Lisbon to Porto is not the most scenic, has the most road walking but also has some interesting towns (Coimbra, Tomar, Santarim, Agueda (just for the umbrella town square), Lourdes). But the walk itself is less grand
I don't know if you've been to Portugal but you will find it different, less touristy, and perhaps nicer than the
Camino Frances. Scenery is not as nice, smaller numbers walking (both pro and con on that one), a little more diverse food (more fish - if that's possible). I would think that March would be very quiet. Along the coast many of the shore side bars might be closed.
The other item that was a pleasant surprise was the number of people who spoke some English. Didn't get much of a chance to practice my meager Portuguese at all. Apparently, Portugal has one of the highest percentages of people who can speak some English in all of Europe.
Enjoy the walk. Bon Caminho