The Miradouro da Senhora do Monte In Lisbon is a great spot to watch the sun go down, or to get some shade during a hot sunny day. I don’t remember seeing drinks or snacks for sale at a cafe like there was at Miradouro Sophia de Mello so it would be a good idea to take your own.
The view over the city of Lisbon from here is amazing. This is the highest point in the city. You can see pretty much everything including Saint Georges castle and because it looks SW you can also catch a good view of the Tagus River.
To get there walk from Alfama, or take tram 28.