Well protected against the north winds, Sesimbra is an excellent place to spend some days.
Pleasant to swim, its good location makes it a good starting point to search for the charms of the region.
Arrábida, Tróia and Espichel are not to be missed, and, when passing in Azeitão don't forget the cheese.
And the wine. And...
A ruined castle on the top of the hill, is a place you must visit, more for the sights then for its own relevance.
It has a history, yes it is interesting, but, believe me, when you reach the walls and see Sesimbra, you immediately forget what is closer to you. The best views are from the walls opposite to a small cemetery, that closes at 6 PM (I think) earlier than the castle (8 PM).
In my last visit we had to try different angles, not so pretty.
A long (here and there narrow) stripe of sand, lined by restaurants, and protected against north wind by the mountain, this beach challenges Nazaré for the title of the best structured beach on western coast.
Nazaré has its people... Sesimbra one or two more degrees in water temperature...
You decide (or visit both... they do deserve it)