Terreiro da Sé SE
Visit the Chuch and Fernandina Wall, after that go down to Mouzinho da Silveira Street and Flores Street, have lunch in one of the many typical restaurants, visit St. Francisco Church or cross the D. Luís I bridge to visit the Port wine cellars.
From this spot one can admire the complete Barredo area, the river Douro and Vila Nova de Gaia.
Cathedral with amazing views over the downtown. Amidst one of the most picturesque yet poorest neighborhoods of Porto.
It is definitely worth spending some time wandering through the interior as well as the outside of this cathedral.