Highly recommended island in the San Juans, about 3 hours north of Seattle. The ferry ride is actually only 10 minutes long - the island is really close to the mainland. But Lummi is special in that there's not much here - a small main village, and then a lot of quiet roads and beaches. We spend 4 days here and really loved it. We took our bikes on the ferry (left the car on the mainland) and then just biked and walked around the island.
We stayed at the Willows Inn, a really good inn with superb organic / local food. The owners really care about the environment and about making this island lifestyle sustainable. We stayed in the cottage up by their farm, and the kids really enjoyed walking through the fields on the way to/from the restaurant.
There's no real downside to Lummi. We were able to fish, go crabbing (we rented a boat and captain for a half-day crabbing a fish outing) and walking and eating and biking. The perfect family holiday.