Upper Lisbon and remote
Most tourists do all their visit in a narrow stripe bordering the river, skipping the upper Lisbon and the most remote areas.
The main touristy attractions are located there, indeed, but to see and understand Lisbon better, you need to enter deeply inland, to see the different areas of the city, and to browse many scattered details.
Highlights in upper and remote Lisbon:
The area of Parliament and Estrela could easily be included in Baixa, section, as the aqueduct could be listed in the west riverfront, however, I think that this is a more comfortable approach.
Visiting this area or not is only a matter of time. Differently from Baixa, here, the few attractions are so spread that demand public or private transport.