We'll have warm weather through the first week in October. Definitely do the Hwy 20 route. Hwy 2 over Stevens Pass is a bit of a mess and irritating, though I do ride it. Here's a nice route to get you from Arlington into Woodinville:
http://www.bikely.com/maps/bike-path...-Century308814
From there, you can take the Sammamish River and Burke Gilman bike trails into Seattle.