Try the Skykomish just above Monroe. There is a classic fly bar that would work. Be very careful in western washington with small trout. Odds are they are steelhead smolts that will one day return (not as 6" creatures) but as brawny 20 lb native steelhead (ok, maybe 8 lb hatchery brats)...but you get the drift.

The lower Green in Auburn is a good spot, as is the Flaming Geyser Park near Auburn. In fact, this is one of my favorite spots. Too bad it is closed now, but if you wait until summer, it can be a lot of fun.