So did you know that it takes 3 hours each way to go from Seattle to San Juan Island on the Washington State Ferry? Well, it does. So I suggest you bring lots of reading materials or jam pack your Ipad with movies and shows to watch. Sometimes you'll get to take a break with some cool sights along the way, like the Puget Sound or Deception Pass. It's a beautiful ride getting there. But don't fret if you missed it the first time, cause you will be seeing the same thing on your return trip back into the city.
Now when I arrived on the island, I obviously needed to explore it! Well, I made the mistake of renting a bike and doing a self guided bike tour of the island. Why, you ask? Because I discovered there are a few steep hills on this island. I thought I was going to die!! Were the views amazing during the ride? Oh my god, yes. But it would've been much nicer in a buggy! Buggies are like $80 vs. bike rentals of $20, so if you have the money to splurge...I would do it!
Another day activity you can do on the island is to visit the Whale Museum. Personally, I thought this museum was very disappointing. It's an old two story house with whale artifacts and marine life exhibits, so I wasn't excited to be in there at all. Rather, I'd recommend just enjoying the scenery and tranquility of being on a beautiful island.