Wonderful place and a perfect time of year to go there.
You haven't mentioned if you're visiting friends, going for work or just winging it. Generally I go for business, but also have friends there, and try to tack on a few days of vacation every trip.
If you're not renting a car or won't have a car for the full time, make sure you check out the TriMet website or Google Transit.
Keep in mind that there is a "fareless square" downtown that allows you to get all over downtown pretty easily just by hopping on any bus. The MAX lines (suburban light rail) goes to spots in NE and NW Portland including the Airport and Washington Park. There are also many transit centers built into the system for feeder buses through different neighborhoods. Overall the system is pretty good, but keep in mind you still might find yourself having to take the bus a lot if you're visiting an area without a lot of light rail (e.g. SE and SW Portland).
Portland had a push to renovate old theaters a few years back, and this seems to manifest itself via many art house theaters, so keep an eye out for little oddball single screen theaters.
There's a large Japanese, Chinese and Vietnamese population in Portland. I'd recommend visiting the Chinatown downtown for dim sum at least one day(I forget where friends took me, so check Yelp). My fave Japanese place is Restaurant Murata in downtown. It's spendy, but very very good. And last, but not least hit Fubonn Shopping Center and Market, if for no other reason than to buy lunch at the deli. It's amazing and cheap, but best reached by car (sadly). Also visit the tea merchant around the corner from the market itself as they will gladly share their tea and their knowledge with you.
In addition to the above, I should mention the Just Be Japanese Pop culture store/Pop Art Gallery/sneaker freaker heaven. If any of that appeals to you, check it out. You may also enjoy Ground Kontrol Retro Arcade as well.
I'll also recommend the Hawthorne district. Very nice small galleries and shops. It's not huge, but good for an afternoon of shopping and cafe lounging.
Powell's is of course a destination, but keep in mind that there are a number of branches all over town with the main store downtown.
If you have a car and can make it out of town, head to Cannon Beach. It's possibly my favourite beach on the West coast now (as featured in The Goonies!). Miles long and very wide sand dunes with huge rock outcroppings everywhere. It's a good day trip. I like to head out in the morning. and spend until about 4pm there.. Then pick up a few pounds of oysters and head back to grill with friends. Totally worth the trip.