Sesimbra is a beach town in Portugal that makes for a great Lisbon city break. The traditional fishing village is positioned in a bay so it's a popular holiday spot for its calm waters (and excellent seafood).
Just out of town you can make a steep scramble down to Praia da Ribeira do Cavalo, one of the most beautiful secluded beaches in Portugal, complete with stunning turquoise water.
Further west on the Cabo Espichel Peninsula you'll find some of the most well-preserved dinosaur footprints in Portugal. (If you're into prehistoric marvels you can discover more dinosaur tracks near Fátima and at Salema Beach in the Algarve and Vale de Meio.)