This august will be hit and miss for boating around here. You will find a ton of things that require a little drive but are well worthwhile. You will have alot more options that are creekish than river play but thats the general truth about later season boating. You get higher up in the drainages, you get less water and you usually get steeper. For your skill range I would suggest: Nooksack and White Salmon but both will be low for the play. The best run to hit would be a combo Nahatlatch / Thompson. The nahatlatch will be pretty low but still plenty fun and the canyon will be easier, while the thompson should be going right into it's primo range during that time. Of course you can check out the Skook too but that is a bit more driving than the nahatlatch and more involved logistically for timing. If you went to skook you could also tap the Elaho on the way in or out.

James suggestion are about the best I can think of... late August it's getting  to be pretty slim pickins, especially in a year with a lower than normal snowpack like we have this year.

Here's a couple more options to note:

The Skagit is a beautiful run that runs all summer due to dam releases. However, it's basically a float with a couple of 2s and 3s thrown in.

http://professorpaddle.com/rivers/riverdetails.asp?riverid=579

The Sultan is closer to Seattle than the other runs listed. It's usually low this time of year, but sometimes summer releases from the powerplant are sufficient for paddling. You can contact the plant operators to find out what the release is forecast to be.

http://professorpaddle.com/rivers/riverdetails.asp?riverid=634

The Suiattle is another option. The rapids are just 2-3, but sometimes wood is a problem, so I'd check here and see if anyone's been on it recently. If we get hot weather in August it could be running as it drains the glaciers on Glacier Peak.

http://professorpaddle.com/rivers/riverdetails.asp?riverid=630


Then again, if we get some freak August rainstorms lots of other stuff could come in (it happens once ever few years) but it would be a real short window. If that happens while you're here you can consider that hitting the jackpot.

Head to Neah Bay, La Push, or Westport on the coast for some kayak surf. 

Basically from Seattle, you're looking at 4 hrs + no matter what. Like everyone said, BC will host the most (options). The White Salmon will be good, though. This year Mt Adams has better snow pack than the rest of Washington.

Yeah, just watch that last ledge in B.D. at any flow below 1,000. Don't flip above it and wash into it upside down-- You will hit rock at enough speed to get potentially hurt really bad. Spread the word.