Here's a list of ballot boxes where you can take your completed ballot in person. You do have two options:
1. Get it postmarked by election day
2. Drop it into a ballot box by 8pm on election day.
Here's where you can find the closest ballot box near you:
Washington State complete list
King
Pierce
Snohomish
Kitsap
Skagit
Thurston
Mason
Jefferson
Island
Because of their local focus, a lot of these races will directly affect your lives on a day-to-day basis more than the bigger elections say for President. Seattle will choose a new mayor (who would possibly want that job?), a few city councilors, and a city attorney.
Thanks for voting!