To visit our island you take a Washington State Ferry from Anacortes. Of all the wonderful things that the government does for us, pre-planning is not one of them, so you will have to plan. Plan for ferry line activities that will inevitably happen in a very big parking lot with a lot of other cars and a lot of other lines. Be sure to arrive at least an hour before departure during the prime vacation months to avoid overload delays.
I suggest a Frisbee because it does less damage to cars than say a football or baseball, believe me I speak from experience. A pack of cards can be helpful as well, however a clingy, drooling dog can hinder you card playing in an already over-packed car.
The Anacortes ferry terminal also features a pet area for dog walking and a nearby public access beach.
For ferry times I suggest you follow this handy happy link to Washington State Ferry Information. The ferry ride over here is a beautiful experience which makes up for any ferry wait.