These beaches are in an area 40 or 50 km north of Lisbon. I don't know the are very well, but I spend one night in a little hotel facing the beach of Porto Novo. I was lucky to be there in time for sunset, and then the next morning for moonset.
A few km south there is the popular beach of Santa Cruz, with its famous rock arch. This stretch of coastline is dubbed "Jurassic Coast", for the age of its rocks, and the many dinosaur fossils that have been found.