One thing I noticed about Seattle when I recorded there was how quickly the density of downtown gives way to calmer, more residential atmospheres in such a small area.
There are parts that are dense, but not anything like New York City, for example. Even though Seattle is a major city it doesn't seem to have an intensity that matches its size.
Now, I'm not a native, but that's the impression I felt.
I have one track of the Belltown neighbourhood you can check out for reference:
It's fairly close to the core but you don't hear the city drone or frenzy of a busy city.
It's also a coastal city -- so, seagulls abound. There are ferries to the islands. Perhaps that could fit in somehow?
It's unlikely that it fits with your project, but one distinctive sound is Seattle's Pike Place market. The fish mongers there sing/chant in unison whenever they make a sale. They also lob the fish through the air to each other before they pack it up. Lots of fun to watch.
Check out the fish sellers at this link:
Great city, by the way.
Best of luck with your project.