On the beach:
Hunt for treasures (shells, sea glass, agates, etc.)
Build a sand castle
Dig a hole as deep as yourself
Fly kites
Play football, volleyball, other sports
Skim Board
Take a surf lesson
Stand up Paddle Boarding
Tide pooling
In the State Park:
Walking
Smurf Hunting (especially fun for 8-and under during mushroom season)
Mushroom gathering
running
Go wading in the wetlands
Climb Washington's Tallest Lighthouse
Visit the Maritime Museum
Visit the Fresnel Lens and the Whale Skeleton
Visit the Coast Guard Station (seasonal)
Rent surrey cabs and ride along the marina
Rent bikes and ride along the beach path or down to Tokeland.
Fishing in the surf or off the Fisherman's Boardwalk
Razor clam digging (seasonal)
Crabbing off Docks (seasonal)
Hot tubbing (at your vacation rental)
star gazing (with no city lights)
Camping at Twin Harbors Campground
Dining at one of the many restaurants
Shopping