It depends a lot on what kind of things you consider attractions. Maybe I can tell you some different things you may visit while in Porto, and you decide if they seem interesting to you.
My experience in Porto: well, I lived here almost all my life, was born here.
Lisbon: Lived there for one year. Completely different. Lisbon is white, Porto is grey. Lisbon has a bright light, Porto seems almost allways foggy! Lisbon has Fado, Porto does not (well, now it does, for tourists).
Things people usually don't do when they come to visit:
- take the metro to 24 de Agosto, visit the water "coffret" found there a few years ago (due to the construction of the metro station), walk to the end of the street and visit the garden of SMAS (SMAS is the water company), great views over the river, and 15 old fountains were moved there, they have no use anymore as water come on pipes, they are beautifull.
- Take bus 508 or 601 and leave at Carvalhido. Admire the beautifull tile wall of Carvalhido chapel, with a clock in one of the towers, and the cardinal points on the other. Cross the street to the garden and look at the "cross" there, from the 18th century, an homage to the pilgrims to Santiago de Compostela (this is part of the portuguese "camiño"). Then walk a litle bit further and visit the biggest iberian labyrinth. To enter there you cross a "gate" in stone designed by the same guy (Nasoni) who did the Clérigos Tower. Then return and have a "Francesinha" at the restaurant named "Rio de Janeiro" - one the bests in town.
- take tram nr 1 (or bus 500) to "Passeio Alegre". It's a garden in Foz, where the river meets the ocean. Admire the lighthouses, but if the waves are big, keep a distance. Have a coffee at "chalé Suiço", in the garden, and admire the parrots over the trees and the ducks in the lake. Visit the toillet (even if you don't need to), you must pay 2 euros to the lady in charge. The sink and all the other things are hand painted in blue and gold, really nice! Walk a litle bit direction city centre, left the garden admiring the 2 columns also designed by the same guy, Nasoni. After 200 or 300 meters take the boat "Flor de Gás" (1,75 eur each ticket") to the other side of the river, Afurada. Nice fishing village. See the old men patching up the fishing nets, and the women washing clothes in the communitary sink (yes, they have washing machines at home, but this is their facebook, twitter, whatever). Have a nice meal of fresh grilled fish. If you are in the mood, walk a litle in the nature reserve and see the lovely herons.
- You would like to see the city from above, but you don't want to climb the 225 steps of Clérigos tower? Make a visit to the D. Henrique Hotel bar. It's on the 18th floor. They have 2 lifts. Orange juice is made of real oranges, and "Alexander nr 1" is great. Or just ask for a glass of Port wine.
Porto is very hilly. It already was at the end of 19th century, so it was decided to build lifts in some of the worst places. Exactly where the funicular (the modern one) is now, there was another one, the first of many that were going to be built in the city. Unfortunately an accident occured, and projects were abandoned. The old machine house is still there.
The old wall: You can walk over it if you enter the gardens of Igreja de Santa Clara (in front of the funicular, upper side). But you can also still see a wall door inside the coffee shop "Porta do Olival", next to "Torre dos Clérigos". Or you can have dinner over a tower of the old wall at restaurant "O Torreão" (expensive and not very good food...). Or you can see that tower, and a part of the wall, at restaurant "Caraças". Unexpensive, very good food, familiar (mother in the kitchen, grandmother at the grill, girls serving at the tables), only 2 dishes and 4 desserts. You eat what is fresh that day.
Hope this helps. Helena.