We just spent a night camping at the state park on the island.
Camano Plaza Market is a decent size grocery (370 NE Camano Dr). Back in Stanwood there's a QFC and Haggen. Next to the Market is a nice kitchen and gift shop.
Checking google before the trip I found Brindles market at Terry's Corner (next to the Islander) http://www.brindlesmarket.com/ It wasn't quite what I expected. They do sell some vacuum packed fish and meats, but mostly they were a restaurant and wine bar. The online menu looks interesting, but we stopped by mid afternoon, so didn't sample anything.
We had lunch at a Mexican place in the same complex (El Morena). The torta was good, though not large by taco truck standards. The chillaquilas were so-so. I can see trying them again.
There's also a coffee roaster in the Terry's Corner complex, with free sample cups, gifts, and internet ordering.