We highly recommend Marcos and Maria's flat. It is in a superb location overlooking the Douro, with a great view of the Ponte da Arrábida, especially at sunset. The flat itself is spacious, clean, and bright. There is a supermarket just 2 minutes walk downhill. You can walk along the river to the centre of Porto within 25 minutes, or take a route uphill towards the Jardins do Palácio de Cristal with its peacocks, views and terraces.
Porto itself is stunning, and easy to explore on foot. Needless to say that you must try the port wine and vinho verde, the former especially in one of the many wine cellars across the river in Vila Nova de Gaia. The beaches there are lovely too.