Oh so very jealous!
Some of my favorites include:
The Yakima near Wymer
There's a fly fishing only lake off route 503 near Cougar that has a great hex hatch that may be around then. Sorry I can't remember the name but the locals should know it - bring a float tube.
Rocky Ford creek
Some second choices:
The Snoqualmie River above the falls
The Washington Deschutes (I don't know if Juro was talking about this one or the better known one in Oregon. It's better known for good reason.)
I've also found the silvers to be agreeable at that time of year around the mouth of the Nisqually, the Tacoma Narrows, and in the Hood Canal around Bangor and Brinnon.
Good luck and I look forward to your report.