Newly opened Paine Field...North of Seatac---Seattle
 

38 miles north of SEATAC airport ..about 1 1/2 hr drive..more if traffic..
Super accessable to Everette. Lynnwood, Bothel...anywhere north or around these towns.
So far "short hop" trips offered by Alaska, United and soon Delta..
PDX, SFO, Denver, Las Vegas .more to be added.
Big discount at present for Alaska air.& others...
NO TSA lines..3-5 minutes..
Short/"long term" parking ...limited and is $ 20 to 30$ day.
Motels --15 min.and i am sure will have "Park/ride" deals later..
Only ONE restaurant/Bar at departure level ...
Everything...fresh and new...
Great airport for those interested in North Cascades areas...
Mt.Baker...or Stevens Pass US 2...or even going on north into Canada..

Wow -- wouldn't that be easy for Whidbey Island, Port Towsend etc.?

Very; it's probably ten minutes to the Mukilteo ferry. If the aim is Olympic National Park, it would also be very convenient, using the (bookable) Whidbey - Port Townsend ferry. The Mukilteo ferry gets crowded on weekend mornings and briefly around the evening rush hour (not terrible for vehicles - big Boeing commuter boats) but still way faster than either the Seatac - Tacoma - Port Angeles drive or fighting traffic to the Seattle - Bainbridge ferry.

There aren't any rental car offices at the airport just yet, but I imagine that issue will be addressed very promptly. In the meantime (assuming they aren't laying on shuttles already) a cab or Uber to one of the nearby rental car offices would be super cheap.

Note if you're an airplane geek or plane spotter, you'll be in heaven as you'll taxi past the delivery line at the Boeing Everett plant.