The oregon dunes are made up of four parts. There is florence, which is the farthest north, Winchester bay, Spinreel, and Coos Bay farthest south. Florence has probably the largest flats, and access to ride on the beach. Winchester is smaller but has lots of big hills, and is my favorite place to ride sand. Spinreel has little open area, but lots of sand trails through forested areas (you can ride to Coos Bay from Spinreel, but it's a long trail with constant whoops). I haven't auctually ridden at Coos Bay but I've been told that it has the largest hill of any of the areas.
There is a noise regulation, and unfortunately they started requiring flags on all motorcycles last year.
And yes the dunes are open year round, they never get very cold, but it is wet. Have fun