One thing that I love about Lisbon is the fact that it is constructed on 7 hills, so you have a lot of points to admire the city from above. Some of them:
Alfama
Castelo Sao Jorge
Elevador de Castelo
Elevador de Baixa
Rooftop in Graca
Miradouro Sofia de Mello
Miradouro de Sao Pedro de Alcantara
Monumento dos descobrimentos
Random street towards Graca