Really hard to find the right place to see dunes. The largest dune near coast is umpqua dunes. Search ‘umpqua lighthouse’, then follow the salmon harbor dr to the end. You can see umpqua beach 3 with large dunes. Lighthouse is good to have a view of all those dunes nearby.
For a first time visitor, it is actually quite hard to find the dunes. We happened across the visitor center in Reedsport, after driving many miles South from Florence. The center was closed, but lo and behold an angel appeared from within and gave us suggestions and directions to two locations that were awesome. One to the South of Windy Bay and the other on the John Dellenback dunes trail. They offered different perspectives but we're both amazing in their own right.
Beautiful place to spend some time. The large loop trail 4.5 miles is quite strenuous since most of it is through soft sand. Near the sign in station a 1 mile gentle grade firm trail goes down to dune level. It has lovely picknic tables with amazing views.
South Jetty - This was a fantastic area for me and my family in our Jeep to discover! Wide enough areas with many levels of dunes. I saw a lot of safe riding and it it was pretty empty for a summer Sunday. I recommend bringing a map if it's your first time out cuz there's a lot of fingers to the dunes here. There were even areas to do some mudding! Be aware of the tide tables if you venture over the last dune onto the beach. Don't forget to air down and bring/mount your flag!
I can’t believe we have sand dunes here in North America, let alone the Pacific NW.