Well, for starters, do not miss the chance to walk around Lisbon a bit and see the sights, it is a lovely city. Walking tour here:
https://www.caminodesantiago.me/community/threads/my-walking-tour-of-lisbon.7257/
My other suggestions for "do not miss" --
Tile Museum on the way out of town, though it does open a bit late for many walkers.
The albergue in Alpriate, run by the Via Lusitana. At 20 km from the cathedral, it's a great first day.
The Convento do Cristo in Tomar, beautiful.
The blue tiled little church in Golega.
The abandoned estate at Cardiga. You'll think you are in wisteria heaven.
The Villa Romana at Rabacal. A few km from town, but the museum staff will drive you out there, and the mosaics are lovely.
The Roman city/ruins of Conimbriga, near Cernache. At 2.5 euros it's a bargain, great mosaics.
Sitting in a cafe in the old town in Coimbra, just enjoying life.
The pig park in Agueda. Up the hill a bit, a shaded forest with lots of colored pig statues all over.
Having a picnic on the lovely shaded grounds in Grijo.
The view (from the high point of Gaia at the park) back across the Douro River to Porto's riverside.
When I walked, there were very few albergues, and there are now a lot, many of which get great reviews (especially Santarem, Alviaizere, Golega, Mealhada, Tomar), but I can't speak from personal experience.
I'm sure I've forgotten a lot, but this is a start.
Bom caminho, Laurie