We stayed in Hotel Britania on the Avenida, and most of these are walking distance from there / the Avenida Metro:
Rubro: http://bit.ly/7KsXd7 - Spanish / Portuguese tapas, including Iberico pork, prawns in garlic, salt cod omelette, cuttlefish croquettes, pimentos. Large, modern, open space. Attentive service. Really good - sadly closed on NYE, NYD and the 2 January, as we'd have gone back for more.
Solar dos Nunes (taxi ride away) http://bit.ly/82aHDJ - traditional, we were taken there by a Portuguese friend. Couvert included liver, marinated mushrooms, cheeses, smoked ham. Grilled fresh fish with potatoes and veg, followed by a wide choice of eggy and almondy puddings. For three, inc two bottles of Alvarinho, bill was €190. We think we had preferential service as our friend knows the owners - but everyone else seemed pretty happy there too.
Pasteis de Belem (tram ride away): http://bit.ly/cEexa for the custard tarts, port, bicas and croquettes. Something like Betty's Tea Rooms but on a vast scale, the rooms just keep on coming.
Ribadouro: http://bit.ly/6Jpb32 for clams with garlic and coriander. Bright to the point of over-lit, but cheerful, friendly service, and a wide seafood list.
Tamarind: http://bit.ly/5FIueC, mild, mainly south Indian food, in a bright pink and blue tiled restaurant. Really good.
For a morning bica and pastry at the counter, we liked Pastelaria Smarta - no messing, order what you like, eat, then pay at the separate desk http://bit.ly/5CRRkp.
Also ate at one of the Cafe Nicolas and Sanchos - both fine, although Sanchos proved a pricey meal.