The Praça do Comércio (the Commerce Square) commonly known as Terreiro do Paço (Palace Square) is one of the most magnificent squares of Lisbon and was once the main maritime entrance to the city. There you have an extraordinary view of the rio Tejo (Tagus river) and both its bridges, Ponte 25 de Abril and Ponte Vasco da Gama. And on the north side, the square is centred by a remarkable 19th century triumphal arch that leads to Rua Augusta, one of the main pedestrian shopping areas in downtown Lisbon. There, you must try the Pastel de Nata (Portuguese Tart).
I´m sure you will adore it. It´s simply delicious! Locals eat it sprickled with cinnamon and powdered sugar.