I've been coming to the San Juan Islands two hours north of Seattle for the last 30 years and each time we try to do something new. Turtleback Mountain is on Orcas Island and a ferry or small plane is the only feasible way to get there if you don't have your own boat (like me). There are several trailheads with interweaving trails over the mountain. We took the South Trail near Westsound.
We have the San Juan Preservation Trust to thank for this hike. It is a private, non-profit and membership-based land trust dedicated to helping people and communities conserve land on the San Juan Islands. In 2006 it purchased Turtleback Mountain for $18.5 million.
The South Trail is a quick 3 mile round trip with only 830 ft of elevation gain. The trails are very well maintained and offer plenty of island vistas along the way. You'll walk through Garry oak woodlands, grasslands, conifer forests, and wetlands along the route and ultimately find yourself at Ship Peak where we took in the sunset over Vancouver Island.
Rather than head back the way we came, we decided to take the Lost Oak Trail down by headlamp. This is a much steeper hike, so if you want more of a workout you could take this trail up and the South Trail down.