San Juan Island National Historic Park
This was one of those places I could have easily seen when I lived near Seattle in the early 2000's but never got around to doing.
The Ferry: You take a ferry with a few different options although Anacortes to Friday Harbor is the most popular and the one I chose. The smart play is to snag your reservation online to make sure you get a space on the Ferry. You can take the ferry by foot, bike, or car - each with different rates (was about $70 to take a car with a single rider). Once you arrive on the island at Friday Harbor, they have small moped/motor scooters you can rent for a few hours to tour around the island on if you didn't bring your own car. If I ever make it back, that'll be the option I go for (assuming the weather is nice).
The Sites: There are two camps you need to visit - the English & American Camps. The English Camp has some walking areas and wonderful views. The American Camp has a larger historical area and the official visitor center. It really only takes a couple hours to tour the island and sites from end to end. However, once you include ferry travel and ferry loading wait times, it's safest to plan a full day or at least three-fourths of a day. (Verizon Mobile welcomed me to Canada while I was at the English Camp!)
Friday Harbor: The locals know what to do with tourists. They have set up a bustling downtown like area right at the Harbor loading dock with great little pubs, eateries, and gift shops. The main eatery right on the water had possibly the best crab cake I have ever eaten - don't skip on stopping there for a bite and a little down time to soak up the atmosphere of the island.