I agree you should check out the Anacortes ferry, if they still have their summer deal for RVs you can save a lot of money.
I also agree with all of the suggestions of what to do,but there are so many more.
One of our favourite places is Buttle Lake campground in Strathconna Provincial Park.
Finally,4 days is definitely not enough, a week minimum would be my suggestion if you can do it. Also,if you are here in the summer, campground reservations are recommended, the popular ones fill up pretty quickly.
I'm sure you will enjoy it, it is a beautiful place. We emigrated here from England over 40 years ago and never regretted coming here.