Porta do Sol, also known as Paquito's (the Spanish owner) is a perfect place to enjoy gorgeous seafood. Not a sophisticated restaurant, but rather a good, good food.
The owner, Paquito, is always around. Ask him for the special of the day. Usually, the must try are the "Bacalhau Debora" (codfish), the shrimps in creamy cheese, the "moqueca" (a mix of several seafoods) and the octopus (don't miss!).
The menu is somewhat more expensive than other resturants, but an entree is enough for 3 persons. So, at the bottom line, it becomes more than fair.
Try also the "sangria", a drink made with wine ans fruits juice.