Last September when we visited Lisbon for three days we spent an afternoon at Costa da Caparica, the largest contiguous beach in Portugal, with an expanse of about 30 kilometres, just south of Lisbon. By bus you’ll arrive right behind the boardwalk, where plenty of beach bars can be found. At the end of the boardwalk there’s a little train, which takes you (for 5€ return) 8 km south along the beach with a total of 21 stops.
There are several ways to get from Lisbon to Costa da Caparica by public transport, for example taking the bus 161 from the Areeiro metro/train station.
We chose to take the ferry from Cais do Sodrè to Cacilhas (Almada) and visit the Cristo Rei monument, that overlooks Lisbon from the top of a hill in Almada. Afterwards we took the rapid bus 135 to Costa da Caparica. The bus leaves about twice an hour from Cacilhas and takes about 30 minutes to get to its final stop right behind the beach. On our way back to Lisbon we got off the bus in Cacilhas and walked along Cais do Ginjal to the restaurant “Atira-te ao Rio”, where we enjoyed a beautiful sunset and a delicious dinner before taking the ferry back to Lisbon.