Walkable, clean, outside of Seattle sounds to me like Bellevue or Kirkland. Both have nice downtown areas that are very walkable - Bellevue's is a little 'bigger' with a huge mall, big buildings, etc. Kirkland has a more laid back vibe than Bellevue, more little bars and there's a park/waterfront nearby. Some of the other suburbs you listed are more car oriented (like Lynnwood, Shoreline, Mill Creek).
Belltown and Columbia City are both within Seattle - both have nice areas you can walk to though. Belltown is nightlife central on weekends though, it can get loud and there is some crime. I know lots of laid back people who live in Columbia City - they have a nice few blocks of cool little shops and restaurants but the surrounding areas aren't so nice.
