I am based on KPAE, but have not lived out here that long. I think you should break these airports up by region, namely what side of Puget Sound or what Island they are. Bremerton and Tacoma Narrows and Port TOwnsend are easy to get in and out and have tradtional transient parking, but are nowhere near each other or near Seattle, if you going to drive from there. It seems that for the West Side of the Sound, airport choice is driven by what is closest to your friends.
For the Seattle side, if you accept that you will be at an FBO and give up your CB status, I think BFI is the best location to get to downtown Seattle including practical public transportation options. PAE is probably cheaper FBO's (my base, so I have no idea the cost) but adds some drive time, and you would need a car or plan a series of public transportation steps to get to Seattle itself. I heard that Renton has limited options for transient parking. Research that one closely.
Port Townsend is a popular restaurant stop and has a small but nice museum. Bremerton is a good restaurant stop. Friday Harbor and Orcas are great day trips because they are otherwise a long ferry ride, so non-pilots do not get to visit these places as a casual choice.
SeaTac does have a GA "terminal" in the SE corner. I have not done it, but you can land (big fee) and park at the FBO (Signature?) for a large overnight fee, and you remain outside of security and away from the real terminal building. I doubt the price is justified when BFI is closer to Seattle and more GA oriented.
Enroute from Montana and need a meal? Felts Field in SPokane had a resaurant that caught fire, but I think they have reopened ( @Ragsf15e should know). Davenport (68S) is a sleepy town with a good Mexican place you can walk to across the street and down a block from the airport.
-dan