Lisbon has lots of great beaches at short distance, both North and South of river Tagus. Some time ago featured here Praia Morena (really a short distance from Lisbon - 15 km). Today, bring to you a place further from the city but that you can include on a day trip, visiting some other awesome attractions nearby.
Arrábida natural park covers more than 100 km2 of beautiful green hills that dive down to a calm blue sea. Here and there, secluded sandy coves are the perfect places to enjoy a summer afternoon.
A couple of weeks ago returned to Portinho da Arrábida (almost 20 years after my last visit). The charm is all there, the water is still cold but so quiet that is impossible not to go for a swim, the beautiful light and the sun setting behind the mountain continue to be enchanting.
However, mind that, if you visit during Summer you must be prepared to walk a few kilometers because the cars can't go right down to the village and is better to take some flip-flops to the water to avoid the pebbles. In the other hand, you can rent a shade on the sand and on the village you'll find some nice fish restaurants.
Portinho da Arrábida is about 45 km South of Lisbon, and you can combine going there with a visit to Palmela, Sesimbra or Setúbal. If you love castles you can find one on each of this places. If you're more on the foodie side, you'll be on heaven too. The sea food, cheese, pastries and wine are of top quality and with very affordable prices. If you want to dedicate the whole day to relax by the sea, not far from Portinho there is another much appreciated beach - Figueirinha.