I lived in Portland until very recently, and I would personally recommend avoiding most of the above advice.
To wit:
Avoid all of the doughnut places - honestly, a doughnut is a doughnut, and it's just going to leave you feeling crappy for a couple of hours during your short trip.
You're not going to get an employee discount to the Nike store if you're an out-of-town visitor, and even if you could, what a stupid way to spend a couple hours of your short trip.
Powell's is pretty cool, but it's not 10 years ago, so stumbling across old books doesn't have the same romanticism it used to. Powell's is like high school reunions before Facebook.
DO: Eat at Screen Door. Get the fried chicken. Get the fried chicken and five other things and send me a picture, please, because Screen Door is God's gift to us. (If there's a wait, get coffee at the place next door, which, GOOD LORD, is some good coffee.)
DO: Eat at Tasty n Sons. If you've got people with you, all the better - it's family style, so order a few dishes and pass them around. (HINT: Bacon-wrapped dates.)
DO: Get happy hour at Portland City Grill. Get there before happy hour starts if possible - best-case is to get there around 4 so you can lock down one of the 20 or so window seats that offer the best view in the city, and $5 drinks. But even if you don't get a window seat, get a seat by the piano because the piano player is awesome - he's a blind kid who plays just about everything from Bach to Skynyrd to John Williams.
DO: Go to the huge food-cart pod downtown for lunch. It's at roughly 10th and Alder, but it takes up a couple blocks so you don't really need an address. I'm not going to give you a specific recommendation, because the whole beauty of food carts is that there are tons of options. I'll warn you that it's the definition of paralysis by analysis though. Once you've grabbed something, go down to the park at 9th and Washington and eat. There's always somebody playing a guitar or a ukulele or something for change.
Here are some other quick ideas:
Salt and Straw, for the best ice cream you're ever likely to have; Tails & Trotters, for the best sandwich from a pork-only butcher you're ever likely to have (get the Banh Mi!); Michael's Italian Beef, for the best hot-beef sandwich west of Chicago you're ever likely to have; Casa Diablo, for the best lap dance you're ever likely to have; Devil's Point, for Stripparaoke; Paadee for outstanding Thai food (the best I've had outside of Thailand; I'm nuts for the Gai Grop Sam Yan).
And, if you ever get the chance to go back with a little more time - I highly recommend a limo wine tour. You'll want to do some googling on that, because the best companies tend to rotate around a little bit for some reason. But it's an A+ day if you can talk yourself into it. For $80 or so a person, you ride around in a limo through some of the best wine country in the world. They drop you of for an hour or two each at three different wineries, where you sit outside and drink incredible wine for incredibly cheap prices (the hope, on the winery's part, is that you buy a case or two, or subscribe to their monthly shipments - but there's zero pressure to do so).
