One of my favorite running spots in the Seattle area is Hamlin Park in Shoreline. The entrance is located on 15th NE and NE 160th.
I was introduced to Hamlin Park during my Adventure BootCamp days a couple of years ago and I have continued to go back for trail running every week, rain, snow or sunshine.
The trail runs through the forest (so I never get wet when it's raining!), up and down hills and is about one and a half miles long. Sometimes I run the trail two or three times but mostly it is just nice for a quick getaway to clear my head. A bonus is seeing a pileated woodpecker or hummingbird!
The trail markers look kind of mystical and the same guy comes to the park almost daily to rebuild what the kids knock down.
Hamlin Park is eighty acres and was renovated this winter with an updated baseball field, children's play area and a more defined walking trail. It is also a popular spot for company picnics, geocaching and sports.