Very nice city, but surprisingly, a lot of traffic and people. But the people were SO nice! We visited in October. I can't imagine it in the summer. The Old Town area is very nice and walkable. Plenty of hills, though.
We stayed in the Porto Palacio Hotel, which was nice, although it's a good ways from anywhere you want to go. (For reference, it's also nearly across the street from the Crown Plaza).
Very nice boardwalk along the harbor to the west of the city. About 2.5 miles if you were to walk the whole thing, north to south. Quite a few seafood restaurants, and others, in the area of Rua de Tomaz Ribero and Avenue Serpa Pinto.
Also a nice boardwalk along the Ribiera, just under the Iron Bridge. There's a tram that will take you to the top of the Iron Bridge and it's just a short walk from where it exits to the bridge. You can get on the tram at the base of the bridge, on the Old Town side of the river. Four Euros, well spent, as it's 218 stairs down.
Letto's bookstore is over-rated, IMHO. SUPER crowded and if you want pictures, unless you're there at opening time or closing time, there will be a hundred people in your pictures. Book a ticket online for a specific time and pay extra to jump the line. Still won't do anything about the crowds inside, but you don't have to stand in line as long.
Looking at Letto's, you can see a church across the plaza to the left. 5 Euros to go in and look around. Interesting place, including the catacombs, robe room, treasury and secret house. Plus you can go up on the roof for a nice view.
The main train station is also worth a visit to see the painted tiles.
Didn't make it across the river to the Port houses, but our friends did the day after we left. They really enjoyed it. Saw a few different Port wineries.
The Majestic Cafe was also worth a visit, but also very crowded. Be prepared to stand in line to get inside. Good hot chocolate.
Castro's has some amazing pastries, which is the only thing they sell, but you'll also likely stand in line to get them. Delicious! So fresh that they'll burn your mouth, so be careful. Not far from the main train station.
All in all, Porto is worth a few days visit.