"I peg the PNW hiking season as early April to late October. In the spring, I look towards coastal hikes in ONP, or lower elevation hikes on the east-side of the cascades. You can still hike in some higher elevations late into October, depending on the weather and how far East you go. There are also some good desert hikes, and hikes in the Blue Mountains of South Eastern WA that offer early/late season options. I downhill ski in the winter and have just started winter hiking/nordic skiing, so there is no real off season me. Add sea kayaking into the mix and there is no reason you can't be busy year round. During the summer season, from July-September, I think WA has some of the best hiking there is."
I would agree with just about everything Edward has written in his post. There is something for just about everybody inclined toward outdoor pursuits, including but not mentioned, fishing, clamming, mushrooming, hang gliding, and more. The one downside at this point in time is that a few hundred thousand other outdoor lovers have already figured this out, with many, many more on the way. Seattle is the fastest growing city in the US today, and it is not handling the process well at all.
I have lived here for 30 years, and am watching with a heavy heart as the hapless clowns of the King County and Seattle City governments drive a once beautiful city and surrounding area into the ditch with uncontrolled growth, neglect of infrastructure, sky high housing prices rapidly becoming beyond the reach of all but the well heeled, and gawdawful traffic congestion that will soon grind the city to a halt. So, be aware that the nearly unparalleled recreational opportunities up here come with a price tag, literally and figuratively. If you are among the well heeled and don't mind congestion and chaos, ignore my post. Otherwise, give your other alternatives some serious thought.