Take the tram from Sintra (the stop is next to the Museum of Modern Art) and ride downhill 13km all the way to the beach of "Praia das Maçãs". The ride takes 45min. and is delightful.
On the way in, on the tram, you can stop at the Adega Regional de Colares i.e. the Colares Winery for a taste of this very rare wine.