Arco da Rua Augusta is a real time commitment — fit it in only if it's a priority.
Allow 20–30 min.
A quick, cheap, central viewpoint most people walk under without realising they can go up — the terrace frames the square, the river and the dead-straight pull of Rua Augusta beautifully. The catch: the top is small with a tight lift-plus-stairs combo, so it backs up, and it's a brief stop rather than a destination.