Since you are starting south of Seattle (Dash Point park), I'd focus on things in that direction. Not an "island" experience, but Mt. Rainier NP is really quite gorgeous. Stop up at Paradise Inn and take in the scenery, maybe a short hike up into the flowers. There's five to six hours of driving if you do the loop around the mountain. Quite different mountain that those you might be used to in Alberta.
Or head out to Pt. Townsend. You can make a loop by going through Bremerton and Silverdale in one direction and taking US 101 in the other.
Or do a loop drive using the Pt. Townsend-Coupeville and Clinton-Mukilteo ferries. If you want the water experience, this is what I would recommend. Ferry route map
here. If your pickup is under 22' long, your total fares would probably be just over $40. See fare schedule
here. Beautiful scenery from the decks while on board. Downside is you will have to traverse Seattle at some point on this route. Perhaps the locals can give you an idea of which direction to do this loop considering both the traffic through the city plus which direction is best to take the ferry at what time of day on a Sunday. Might be worth doing in a counterclockwise direction to avoid the lines at the ferry terminal from those returning from their weekend on the Olympic Peninsula. Plus this gets you through Seattle earlier in the day on a Sunday.
Ken & Allison
2 Camping Cats (1 diabetic)
1996 4Runner, TRD Supercharger, Edelbrock headers
2007 Fleetwood Arcadia, Honda EU2000i
4 mountain bikes, 1 canoe, 4 tents, 8 sleeping bags, 2 backpacks
(You get the idea!)