Roughly three hours by car from Lisbon, the beach is free to visit, but it's one of the more popular spots in the area and not huge, so it can get crowded pretty fast (get there early in the day if you can). To access it, you'll park in a nearby lot then descend a long flight of stairs and a man-made tunnel, which drops you out in a cove surrounded by tall sandstone cliffs.
It's a perfect place to simply kick back and chill by the shore, but the real standout features are found up along the cliffs. There are lots of hidden and secret paths to explore, including one that'll take you to a secret beach inside a cave, which you'd otherwise have to take a boat or swim to. You'll also find lots of winding paths along the edges of the cliffs nestled with special spots from which to face your fear of heights and dive in (don't worry the water below is plenty deep).
The only downside -- if you can call it that -- is that there aren't any restrooms, restaurants, or bars anywhere nearby, so just prepare to come prepared.