I've lived in Seattle for 50 years. Southern BC and Washington are truly twins, so you'll mostly find variations on BCs many treasures down here.
I'll be going up to Victoria and Sooke in a couple of weeks. A great place to have in our back yard.
In the Olympics, the Hoh rainforest and Hurricane Ridge are worth a visit.
Farther inland, Mount Rainier is a must. No pictures come remotely close to seeing it close up, or better, hiking on its many great trails.
Seattle is also a near twin of Vancouver (although Vancouver is still more beautiful, imo), but it has been undergoing a huge transformation over the last 15 years. Interesting to see a boom town in full boom mode.
Many don't know that Oregon, Washington, BC and Idaho were once all known as Columbia. After Canada was established, your part became known as British Columbia. Washington and Oregon became Oregon Territory.
When Washington applied for separate statehood, they asked to be named Columbia, but a committee of Congress decided it would be confusing to have DC, BC, and Columbia and named it Washington instead.
If you're coming down this way, send me a PM, we can have lunch, listen to tunes and talk about this stuff.
Welcome!