I'm with Kathryn, I went to Nazare for a few hours and that was the only place I can say I wish I hadn't bothered and didn't enjoy at all. My trip was in May-June this year, 23 days, and I skipped the Algarve, still didn't get to all the places I wanted to, though I did have a car for over a week, mostly for the eastern side of the country.
I have enjoyed many days along the northern California and Oregon coastline. After that, the bits of Atlantic coast I saw in Portugal didn't awe me. I did enjoy walking out to Boca da Inferno while I was in Cascais and watching the sea pounding the cliffs.
Sintra is much better if you don't try to day-trip there from Lisbon. It's worth 2-3 nights. You can day trip to Cascais from Sintra or Lisbon. I went from Sintra because the hotel there was a lot cheaper than in Lisbon :-). I took Uber back at the end of the day and it was not expensive, well worth the price.
Consider doing Evora as a day-trip from Lisbon. It's about 1.5 hours on the train. If you make it an overnight, you have to go back to Lisbon anyway to get a train to Tomar or Coimbra. I used Tomar as a base to visit the beautiful monasteries of Batalha and Alcobaca. I believe there are day tours from Lisbon, but Tomar is a lot closer. There may be tours or bus service from there. If so, you could spend 2 nights in Tomar and see a bit of the old town as well. 2N in Coimbra gives you a full day there, which is enough. I spent a night in Aveiro on the way from Coimbra to Porto (by train) and enjoyed it very much.
I stayed 2N at Quinta de Maroccos in the Douro Valley and absolutely loved it, the place ,the decor, the staff, the dinners, the winery tour and especially their wines. I had a car so I can't tell you how easy it is to get around without one.
Lastly, I'd recommend at least 3N in both Lisbon and Porto, more for day trips.