Portland has a host of fun neighborhoods for window shopping, people-watching, and drinking and eating (and drinking) (and eating). As eastsiders, we'd especially recommend Division (from SE 20th to SE 48th or so), Hawthorne (from SE 30th to SE 50th), and Mississippi (from N Fremont to N Skidmore), but you'll also find plenty to occupy your time on NW 23rd or in the Pearl District, which are beautiful (and a little swankier).
We love both the Portland Japanese Garden and the Lan Su Chinese Garden (and the various Rose Gardens around the city, but those won't be in bloom in September, sorry to say). If you have an hour to kill, head to the Chinese Garden - if you're looking to fill a long, spacious afternoon, the Japanese Garden is for you.
Powell's City of Books is just a few blocks from the Sentinel and a Portland icon for a reason! It is far too dangerous for us to go inside there anymore, especially Annika, but we recommend the place wholeheartedly to all the bookworms in our lives.
Explore Bridgetown from a new angle! Great for outdoor enthusiasts who don't want to leave city limits. Easy rentals and water entry at portlandkayak.com.
There's no place prettier than Mount Tabor on a summery day - and don't worry, it's extinct. Hit up the Bellwether Bar nearby for a post-hike refreshment!
If you don't mind a long, winding drive into Clackamas County, Bagby Hot Springs is a gorgeous (recently-reopened!) spot for a short hike and a contemplative soak deep in the forest.
At Silver Falls State Park, you can visit 10+ impressive waterfalls along a 7+ mile loop, including several grottos where visitors can walk behind the falls. A great day trip if you are planning to spend extra time in the region!
Through Sunday, September 15, Portland Art Museum will be home to 'Monet to Mattisse: French Moderns', showcasing approximately 60 modernist masterpieces. (Bonus points if you follow up with a visit to a Portland patisserie or wine bar - let us know if you need a recommendation.)