Weather should be pretty good. August is generally one of our hottest months (upper 80's-mid 90's) without much humidity.
Probably won't be snow on the mountains so snowboarding is out, but some of the most beautiful hiking or mountain biking you've ever seen.
The Gorge is world famous for it's windsurfing; plan a day trip to Hood River (1 hr from downtown PDX) windsurf all day and dinner @ the Full Sail brewpub. Stop on the way to check out Multnomah Falls.
Oregon wine country (Dundee) is also only about an hour from downtown. 8 or ten good wineries in a 1 mile strip (Argyle is a personal fave.) Spend the day doing tasting tours, or head down highway 99 a little further and check out the Spruce Goose at the Evergreen Aviation Museum.
You could easily spend 3 or 4 days in Portland itself checking out the sights. Several fountains and parks; the Pearl District for art; Powell's bookstore can be a day itself; OMSI, Saturday Market; Underground Portland tour, etc... There is probably a pub crawl every night (we take our beer very seriously). Portland is an easy city to walk. It's well laid out (grid fashion), it's not very big and has some excellent public transportation.
Time permitting head to the coast (2 hr drive) and do some whale watching or deep sea fishing, or take a jet boat ride down the Columbia river to Astoria.
Depending on when things come together, you might find yourself on the PNW leg of the SOHC rally!
-JP
(Ahh, Kit, you beat me to it!

)