Douro River is the heart of the city beautiful scenery.
The uneven slope of both margins offers a great view of the landscape and of the city.
You can search for the best spot to admire this view either from Morro Garden in Gaia or from Ribeira in Porto, take your pick.
Also don’t dismiss the Douro valley tours tha can show you the Port Wine Vineyards up the river where you can enjoy a more rural sightseeing.