There's a few cities where you can ride an historical tram, Porto is one of those magic places.
From the city center you can take it and go to directly to the Sea Front, in a very panoramic and slow ride.Take your time and enjoy the view.
There's also a museum, Museu do carro Eléctrico, where you can learn their history.It's on the way to the sea and you can buy a combined ticket of tram+museum.
Tickets: 2.50€ one way trip, 8€ full day ticket - ride as many as you want.
You can buy your ticket on the tram.