My favorite "low impact" choice would be Deception Island itself- but it may be a wild life preserve, so, then again maybe not. I'd rather be "low Impact" on someone's private prop than disturb my wild friends. North along the shore from Deception Pass may afford such an opportunity...
but without making a crossing, that's about all I can think of, unless you want to camp at Deception Pass State Park with all those High Impact campers who like to bring boom boxes, generators (you NEED one if you plan to watch your plasma screen in camp). Basicly you'll be camping in a forested parking lot among lots of ungracious ilk, and you won't get any sleep...
I say go poach, let this thread die, and have a great experience.