Had the opportunity whilst being stuck here for a couple of years during covid to see quite a lot of the costal areas between Lisbon and Porto (although Portugal's' geography of over a 1000 miles of coastline).
Stand out for me of many stunning spots is Ericeira which is a great town not too far up the coast from Cascais. Really bustling old style authentic town with lots of eateries and bars in the cobbled streets leading down to one of the many the beaches nearby. If you get a chance stay at this hotel (Vila Gale) for a few nights which goes out into the sea and has a really Portuguese colonial feel about the whole place.
View attachment 74172
But you literally have hundreds of miles of pristine beaches and towns all the way up to Nazare (great vibe there especially if you like surfing) up to. Cascais has been spoken about alot, it's whats known as the Hamptons of Portugal and your well to do Lisbonites go there so can be pricy accommodation wise but around 30 minutes away from Ericeira.
For something different the Douro valley near Porto is very nice, had a great week there kayaking, eating and swimming in the Douro river with spectacular scenery snaking all along - wine and Port is not too bad either ;-)