There are no good beaches close to the historic city centre in Lisbon, The nearest good beaches are at Carcavelos, and Estoril on the Cascais Line along the Tagus and at Costa Caparica on the South Bank of the Tagus. (Car recommended.)
The Great Western Riveira in Carcavelos might be off interest. See:
http://www.tripadvisor.com/Hotel_Review-g780694-d1099391-Reviews-Riviera_Hotel_Carcavelos-Carcavelos_Estremadura.html
It is about 5 minutes walk from the beach and 7 minutes walk from the Railway Station with trains on average every 15 minutes into Lisbon Cais de Sodre Station (25 minutes distant) and also to Estoril/Cascais.
