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 6Pm (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)
Cabo Espichel is a rocky cliff near Sesimbra, with good sights over the sea and a sanctuary dating from the 18th century and surrounded by several constructions in different styles.
Pilgrimage, now abandoned, justified a local inn, still standing near the complex "Casa da Agua"