You should always be prepared for rain when coming here. Like Seattle, Portland is known to be one of the cloudiest and wettest cities in the US - not because of the amount of rain it gets, but because of the number of days with rain.
Portland is a wonderful city to walk trought, so pack ther right pair of shoes.
Locals swear on Birkenstock sandals or Dr. Marten shoes, depending on the season.
In the summer, it gets pretty cool at night, so rimind to always bring a sweatshirt with you.
In the winter, layer up. Often, the clouds will part and you will suddenly be under direct sun.
There are tons of unique aspects of the city that you will want to take back home with you. So don't forget your photo equipment.