Praia Grande is probably the closest, and certainly the mostly easily accessed, beach from Sintra. The name translates to "Big beach" and whilst the beach isn't massive, it is the biggest on the Sintra - Cascais coast, rivaled only by Guincho in terms of size. The expanse of golden sand is big enough to hold the summer crowds who flock here, many coming straight from Sintra on the tram which stops close to Praia Grande before continuing on to Praia das Macas.
As you'd expect for a popular beach there are plenty of facilities such as cafes, bars toilets and plenty of parking.
Praia Grande is well known as a surf spot and it can indeed get good here. It is a good choice when the swell is fairly small and the high cliffs provide some shelter from the wind towards the southern end.
At low tide Praia Grande joins up to neighbouring beach, Praia Pequena (Little beach). Between the two beaches is the huge outdoor saltwater swimming pool belonging to the Hotel das Arribas. At 100 metres in length it is one of the biggest in Europe.
Praia Grande is incorporated into the Parque Natural de Sintra-Cascais and the beach has a wilder feel to the south where huge cliffs rise up from the beach. These cliffs are home to an interesting set of fossilized dinosaur footprints which appear to walk straight up the near vertical cliff face (the cliffs have moved over the years!). Dating back around 100 million years there are over 60 imprints which are believed to belong to Megalosaurus and Iguanodons.