The northern boundary of the Oregon Dunes National Recreation Area [1] is at the South Jetty ($5 per car or Northwest Forest Pass [2]), where the Siuslaw River flows into the Pacific Ocean. May–September and on all weekends and holidays, the beach at the South Jetty is closed to motor vehicles, and even though there are no marked trails, it’s a great place to explore the dunes in near solitude.
The road into the jetty has several staging areas for off-highway vehicles; during the summer months, the area south of the road is open to motor vehicles. South Jetty Road is 0.5 miles south of the Siuslaw River bridge.
Links:
[1] http://www.moon.com/destinations/oregon/discover-oregon/explore-oregon/oregon-dunes
[2] http://www.moon.com/destinations/oregon/background/sports-and-recreation/user-fees-and-passes