So much to see in the fantastic Pacific NW.  Here is a sample itinerary.  It is hardly all encompassing, just one idea.

The number of days depends on your time and schedule.

This is a circle route, which can be taken in either direction.

Seattle: 2 days

Whidbey Island: 1 full day  Mukilteo Ferry to Cllinton.  You can spend several days here to see everything.  See Whidbey Island Inside Page.

Olympic National Park - A scenic ferry and drive to Port Angeles at the base of Hurricane Ridge.  Park entrance outside Port Angeles.  Drive a very good and scenic road 17 miles from sea level to mile high visitor center.  Fabulous views of the Straits and mountains.  Very nice walks and trails.  Deer, Olympic Marmots, Bear, etc.  Plan 1 full day for this area, including Lake Crescent, Marymere Falls, Sol Duc Hot Springs, et al.

Port Angeles is the terminal for the Coho Ferry, which you ride to Victoria, on Vancouver Island, British Columbia.  You can either drive or walk on this ferry, but best to make auto reservations.  Victoria does not require a car.  1 day, but rushed; overnight is better; 2 days

Ferry from Victoria (Sidney to the north) through San Juan Islands to Anacortes and then either drive to Seattle or use that day to head south down Whidbey Island; 1 or 2 days, depending on choices you make.

Option:  You could stop at Orcas Island or Friday Harbor, on San Juan Island, via the ferry. Worth 1 full day, at least for either. See Inside Pages.

With this route, you are getting spectacular views and interesting places evocative of the PNW.