Lisbon’s dark, brooding little brother has so much more to offer than people give it credit for. They seem to take things a bit more seriously here. The food, the wines, the (predominantly) religious attractions just seem so much more important for the people who live here. It’s smaller and easier to see in a day, but hanging around for a few helps you get a better feeling for it. It’s also helps you pick and choose when you are going sightseeing in between the bouts of heavy drizzle.
The city essentially breaks out into three sections that the average tourist will want to see.
First is The Ribiera - the older working section along the river full of the city's older, more atmospheric buildings.
Second, Vila Nova de Gaia, the area across the river that houses all of the cities Port Wine Quintas [the tour and tasting at Churchill's is excellent] and some nice restaurants with spectacular views of the city. Definitely head to the top where the metro crosses the river for beautiful views and photo ops.
Last is the Baixa or upper town of Porto where much of the shopping and entertainment happens. This is the area that houses most of the tourist attractions like the Cathedral, The Palacio De Bolsa, Saint Francis Cathedral and that irritating bookstore where they charge you 5 euros to get in because some woman wrote a book about wizards and was influenced by it. See the other stuff. Skip the bookstore.