Miradouro de Santa Luzia ("Santa Luzia viewpoint") is another classic viewpoint in Lisbon. Is overlooks the neighborhood of Alfama as well as the Tagus river.
One of Miradouro de Santa Luzia's highlights are the couple of tile panels featuring historic images of Praça do Comércio ("Commerce Square"), before the 1755 earthquake, and of Christians attacking the Saint George's Castle in the context of the conquest of the city of Lisbon against the Moors in the year 1147.
Miradouro de Santa Luzia is very close to Miradouro das Portas do Sol, which should be your next step from here. Both are just 10 min away from Castelo de São Jorge ("Saint George's Castle"), one of Lisbon's most important attractions.