If you get your fill of Oporto, take a side trip down to Coimbra. There are trains hourly, cheap. Departs Vila Nova de Gaia Arrives Coimbra-B
Check out the university, especially the library. Biblioteca Joanina (google it, look at images)
Wander the old town, check out the church that is half cafe. Café Santa Cruz
Go to the Hotel Astoria down by the river for lunch or a drink great art nouveau style building.
Great vibe in town when college is in session. Take a meander up Rue Quebra Costas (the backbreaker).
This seems a bit redundant to tell a lesbian, but, wear sensible shoes. Coimbra is a walking town and built on hills.