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Both
Lakedale and Roche Harbor are both on San Juan Island. Both resorts have enough happening to keep your whole family entertained.
They have the added benefit of having things to do on site as opposed to off site.
Roche Harbor has a swimming pool, tennis courts, really good kids playground, kayak rentals, and all sorts of activity in general. Roche Harbor usually has decent live music playing in the MacMillan bar nightly.
Lakedale has camping, swimming, fishing, and canoeing. It is situated halfway between Friday Harbor and Roche Harbor.
Snug Harbor is also on San Juan Island to the south of Roche Harbor and to the north of
whale watch park .
whale watching and
kayaking trips leave from snug harbor multiple times daily. You can buy some turkey leg and rent a crab pot then row out and set your crab pot ( ask management where the best spot currently is ). Last time I checked they had a canoe for rent as well as some other smaller dinghies for exploring around the protected Michel Bay.
They have camping spaces as well as small vacation rental type cottages. They also have a small general store with wireless Internet on site.
A shared barbecue area and picnic tables sit at Snug Harbor's edge.
Here are some other places on San Juan Island that your kids will enjoy:
The Whale Museum The San Juan County Fair August 12-15, 2009
The West Side of San Juan Island The movie theater ( downtown Friday Harbor )
The bowling alley ( downtown Friday Harbor )
The Friday Harbor skate Park, ( at the fairgrounds )
I hope that helps... let us know how your trip went after you get home!