Lisbon is a beautiful city with many great viewpoints (miradouros), but I believe that the Miradouro de São Pedro de Alcântara is probably one of the best. The extended view over Downtown Lisbon, the Castle and the river is breathtaking and the garden it self, landscaped on two terraces is a delightful place to rest an have a refreshing drink.
Last time I went there besides the usual kiosk there was a small market with food, crafts and a spectacular sangria. Across the street you can find the bar of the Port Wine Institute where you can sample this famous Portuguese drink.
You can get to Miradouro de São Pedro de Alcântara walking from Príncipe Real or from the Largo do Chiado. If you are Downtown, you can get the Glória funicular up the street (ticket 3,60€).