Tram 1 of Porto.
Passeio Alegre> Infante
Line 1, « Infante-Passeio Alegre », follows the river Douro to reach Foz do Douro. This trip in an authentic charming tram is full of romance and a great way to discover in passing the Solar do vinho do porto (the wine institute), the Museo Carro Electrico and some splendid buildings and landscapes. A tram comes along every twenty minutes or so and they run between 9 in the morning to 5 in the afternoon.
You can also reach Foz do Douro by bus, so take it easy. A taxi fare to central Porto will cost about 10 euros.