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I'll either be in downtown, or out by the airport.
If you're by the airport, you can take Light Rail downtown for a couple bucks (red line goes from the airport all the way downtown). Portland has a pretty awesome and incredibly useful light rail system -- it's an awesome example of what that kind of transportation can be like when you have a metro area that's really dedicated to making it work.
If you have an afternoon to yourself, hike Forest Park. You can take the train to the zoo (the stop for which is in a tunnel) and then take an elevator up to the zoo. There's a forestry center museum, but it's pricey -- more fun is just to hike the trails back down into the city.
If you just have an evening, go downtown and just wander southwest and northwest parts of the city. There's all kinds of neat urban parks and landscapes. I can't think of a city that's better for wandering in than Portland.
I lived there for ten years. Quite a few things I loved about the city, but living there was difficult.