go up the Clérigos Church...
by Scarface
go up the Clérigos Church Tower and enjoy the view of all the city. It's the highest point in Porto, and when there's a clear weather, you'll feel that nothing else is important anymore... The sidewalk along the seaside, near the mouth of river Douro. It's one of the most pleasant experiences in Porto, just enjoying the waves hitting the rocky coast.
A head?
by AndyRG
Inside a wonderful park (you can see even some animals, if there is no fog - as the time I had been), you can have a pleasant walk. Portugal's 2nd biggest city has a lot of places of interest, as the Stock Exchange Palace, the Cathedral, Museum Soares dos Reis, Casa de Serral etc. But the best is to be at the narrow streets of the historical centre, a world heritage (UNESCO).
Tripe
by evilprebil
If you come to Porto, someone needs to try tripe. People here are referred to as tripeiros, or tripe eaters. It is rich in tradition that Porto's inhabitants supported Henry the Navigator by giving up all their meat for expeditions and settling for the tripe. As Rob said, it looks like stomach lining, but it doesn't taste like anything other than the spices. Two good restaurants for this are Abadia and O Ginjal.
porto air race.
by cachaseiro
the porto air race is also called the redbull air race as redbull sponsors the event.
it´s held once a year in early september and is quite a spectacular event with planes flying under the bridges of porto and around town in general.
this is something for people who like the sound of engines as the show is extremely noisy.