If you like sun and want to feel like you are three hours away from a busy city you have to go to Praia Morena in Costa da Caparica or Guincho, just after Cascais. They both have great beach bars with good food and nice music.
Morena Beach is south of Lisbon. If you rent a car it's very easy, it only takes 20 minutes otherwise you will have to get in public transports and that will take a little over an hour.
To get to Guincho from Lisbon just take the train to Cascais, which goes along the river Tejo and then the ocean. Once you get to Cascais, get in a cab as ask to go to Guincho. It shouldn't be more than 10-15 euros. If you have time visit Cascais, a very pretty town just by the sea. Once again, if you have a car will only be 30 minutes driving.
You can also go out at night for a drink, dinner and dancing in "Bairro Alto". I'm sure you will meet someone there who can give you a ride! Lisboetas are crazy for the beach!
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