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I can't stress this enough and I will continue to say that seeing a city by foot is the best way to see it. I love to walk, and I always walk around in every city I find myself visiting. Portland is no exception. It is a very walkable city and if you do get tired, hop on a Max. You can start your visit down by the Waterfront and walk around China Town, then around the Portland Saturday Market (if you are here on Saturday) and around to the Mall and the center on the city.
Make sure to have some good walking shoes.