If you're going to do a guide trip, I would go to the Yakima without thinking twice that time of year. Red's, The Evening Hatch, Worley Bugger, Tamarack Guide Service, lots of options.
You could talk to Creekside Angling Co in Issaquah or Pacific Fly Fishers in Lynnwood for local recommendations, but the Yak is our only "blue ribbon" fly fishing river. The fish are chunky and there are lots of them. It's less than 2.0 hours from Seattle.