As a resident off and on all of my life to Washington State, I have to say that there are many hidden gems here. As to where to go and where to avoid, well that is a matter of personal experience and taste. Some of the reasons I love this place though are the following:
Mt. Rainier and the Tattoosh Mountain Range (grew up with this virtually in my back yard)
The Hoh Rainforest.
Pike Place Market.
Mt. St. Helens.
But, again, this is my list, and may not appeal to others.