It's a very walkable city, but very hilly. The tram/metro is cheap and good though.
My favourites:
Pastel de Nata at Fabrica de Nata, also Nata Lisboa further up the same street is good too. Don't miss the murals on the chapel near there (Chapel of Souls?).
Coffee/ breakfast:
Combi Coffee
Bird of Passage
Esquires
Mesa 325
7groaster (Gaia)
Ice cream:
Gelateria Portuense
Wine/Wine and cheese:
Bacchus Vino.
Vinoteca Gartafeira and Wine Bar, R de Mouzinho da Silveira 88.
Prova
Port:
Augusto's (Gaia) Only 5 Euros for a Port tasting of 4 ports. Family owned, so smaller and less corporate than the big Port wineries in Gaia.
Seafood:
Locals said to try the restaurant at the Mercado (market) at Matosinhos (metro tram goes there). You buy your fish in the market and then there is a restaurant there (Mercado Food and Drinks) that cooks it for you. Unfortunately I ran out of time, so did not get a chance to do this....
Restaurants:
Arco das Verdades (great view). Tapas, but you can also go for coffee and cake or breakfast.
Bota&Bura.
Mistu.
Tour:
Worst Tours. (That's their name!)
Viewpoints:
Miradouro da Serra do Pilar.
View of city from Gaia riverside, especially around sunset when the lights come on (opposite bank of river) and from gardens behind cable car top station (metro stops here or walk across top level of the bridge).
Miradouro da Vitoria.
You can open Google maps on your phone and search for any of these places then tap save to save then to a list. You can also then 'download maps for offline use' to avoid using data.