Don;
Is that what the little yellow thing is about?
We took a couple of weeks in Sept to go boating in the southern gulf Islands in 2008. Constant sunshine, fair winds. Wonderful time to cruise here. Unlike Marin's experience, we found crowds. Over 100 boats in Montague harbour on a Sunday night in mid Sept! Those retirees are taking over!
If you get the chance, cross over to Saltspring Island. many B&Bs so you can stay for as long as you are enjoying yourself.
I woud take the Anacortes ferry to Victoria and then from Swartz Bay to Fulford on Saltspring. Leave Saltspring by Vesuvius to Crofton and drive up to Nanaimo. If you have time to go out to the west coast, go to Tofino. then back from Nanaimp to West Vancouver, see Vancouver before you drive back to Seattle.
We have a house on Saltspring, where we spend a lot of our liesure time, so if you decide to go there, I would be happy to give you some tips on local attractions.