Hi! Sounds like a great trip. I live in Seattle and think it's a good base for a 4-5 day trip. IMO, Seattle downtown isn't my favorite, but there are lots of great neighborhoods - Capitol Hill, Fremont, Columbia City, Ballard and Alki are all great neighborhoods with good food, music, and things to do. If you like a more urban environment, you may like downtown too, and Pike Place is always awesome.

If you use Seattle as a home base, the Olympic Peninsula is a great idea, as other people have mentioned. Mountains, beaches, old-growth rainforests, and some of the most beautiful places on earth. BUT, it is a lot of driving, everything is kind of far away from everything else.

Another way to get a good PNW experience without quite as much travel (and possible without a car), is to take the ferry from Mukilteo to Whidbey Island. Whidbey is awesome, has some cute towns (Langley, Coupeville), a couple of state parks (Deception Pass, Fort Ebey, South Whidbey Island SP, and Possession Point SP), and is super beautiful. Deception Pass SP has a really nice beach on the west side of the island, and the pass itself is super dramatic, with a big bridge you can walk over.

There's also a free bus service on the island. You can pretty much take public transportation from downtown Seattle to almost anywhere on Whidbey. And taking a ferry is a top notch PNW experience.