For the bibliophiles among us, and even those who aren't yet addicted to books, but have that proclivity, you have to see Powell's while you're here. It's the largest independent bookseller in the United States -- and for good reason.
It's not called a "city of books" for nothing. The main store (there are five or more at present, most of them satellite stores) is located on Burnside, housed in an old warehouse. Three solid floors, taking up a -full- city block, with nothing but shelf after shelf of new and used books positioned side-by-side. Prices are great, and they do take trade-ins for cash, if you're looking to unload.
The literature for the store says, only partly tongue in cheek, that if you're stopping in for the first time, it's advisable to pack a lunch. It's easy to get lost in the stacks and not realize that you've missed the meals for the day -- and it's true. Hours pass like minutes while your arms start to ache with the loads of books you didn't really intend to buy, for much less money than you thought you'd have to spend.
There is an online version of the store at http://www.powells.com, though it has none of the charm of the actual location.
Powell's is definitely one not to miss. Your inner bibliophile will thank you.