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.
One of the reasons why visitors don't need a car in Portland is that shortly after the city was founded, early developers decided that city blocks should only measure 200 x 200 ft. While some people enjoy saying that Portland's founders could foresee how "green" Portland would eventually become and therefore made plans to create a pedestrian-friendly city, what truly motivated their decision was that shorter blocks made it possible to sell more corner lots, which of course were worth more money. But no matter how they came to be, these short blocks set up in a grid pattern (for the most part) do make it very easy to get around on foot in this environmentally-friendly city, as do the signs posted at most intersections that indicate how to reach Portland's attractions and neighborhoods (so you don't even need to bring a map!).
I've heard that in pretty much every other city 12 blocks equal (roughly) one mile. The blocks here are small, however, and 20 blocks equal a mile. If you're planning on visiting and aren't going out much farther than the downtown area, consider leaving the car at home and just walk the city. Or combine walking with public transportation and you can get to just about anywhere interesting.
There are also guided tours available. You can learn about Portland art or architecture or the city's history. The phone number and link can be found below for Portland Walking Tours.
Flying is recommended if your coming from somewhere really far away. If you have access to a car then driving is also a good way.
Once in Portland travel by car is good for some of the outlying area's, the zoo, rose garden and other areas. But if your in downtown Portland the only way to fully enjoy the city is by walking it. I recommend you pay for your car to be parked all day or that you ride the Metro into downtown and then walk. The bus system is also fairly good, least it was the last time I used it.
There is a series of maps published by Mapclicks that are terrific. These were even more useful than my Fodor's guide!
They are broken down by different neighborhoods (The Pearl, Nob Hill, etc.) and are the easiest to use with good area advice, too.
These maps are FREE and available in most hotels and also at the Pioneer Square tourist/Tri-Met transit info center.
There is also a very good map of the Downtown given free by world-famous Powell's Bookstore.