Probably my favourite viewpoint. You can admire the centre and the castle in Alfama from here. There is plenty of shade too which refreshing after the walk up the hill.
Hard to choose the best viewpoint in Lisbon, but São Pedro de Alcantara is definitely one of the top ones. It's like sitting on a terrace with a view of the eastern side of the city. There is a kiosk where you can get some refreshments.