Another option if you're just going to Friday Harbor is to park your car at the ferry terminal in Anacortes and walk on. It will be less expensive and you won't have to worry about car lines going out or coming back. As mentioned previously, weekends can get very backed up, especially returning to Anacortes. Because they ferry stops at several islands, they limit the number of cars that can get on at each island and you may have to wait through several ferrys to get back. As far as I know, they don't limit the number of walk-ons, so you can take whatever ferry fits your schedule.