Stop in Fatima and visit the nearby Batalha Monastery.
I spent 6 months in a smaller town called Santa Maria da Feira and loved it. There's a cool castle there on a hill above the town.

Check out Nazare. It's a beautiful village known for its huge waves.

I love Porto, but it's far from a small town. If you stop there, I'd take drive across the Dom Luis Bridge between Vila Nova de Gaia and Porto and check out the shops on the Porto side.

If you want a view of a great Portuguese Catholic church, I'd go to Braga and check out Bom Jesus.

I could go on and on, but I'll stop right there. The whole country is beautiful. I'm 100% American, but I'd gladly live in Portugal full time if I ever get kicked out