The John Dellenback Dunes Trailhead is a day-use park near milepost 222 on US 101. It's a lovely spot, run by the U.S. Forest Service as part of the Oregon Dunes National Recreation Area. It offers parking, pit toilets (no water), and access to an epic 6-mile hike across an expanse of Oregon's picturesque coastal dunes. The dunes here are taller and more impressive than at many other locations, and the expanse of them is inspiring. The hike also encompasses a forested deflation plain with a boardwalk above the swampy bits, and a stretch of beach that is populated only by those who've walked three miles across the dunes to get there.
This is a fee site ($5 in 2018) or use your Northwest Forest Pass. The drop tube for the fee envelope is in Eel Creek Campground. which adjoins to the north via a loop trail or via the highway.
Dina Pavlis featured this spot as one of two hikes to convey the flavor of the Oregon Dunes to visitors in her book, Secrets of the Oregon Dunes. If you are a fan of the Oregon Coast, you should check this out.
Read more at Secrets of the Oregon Dunes, 100 Steps, Alltrails.com, Oregon Wild, Oregonhikers.org (campground loop), Oregonhikers.org (trailhead), Outdoor Project, Siuslaw National Forest, and Google Image Search.