The Stovepipe Wells dunes in Death Valley are fine and easy to get to. Also, more heavily visited and photographed than other dunes. Watch out for cold rattle snakes in the morning light. Closest I ever came to getting bit.
Try Eureka dunes, a new addition to Death Valley. Eureka Dunes are accessable by most standard vehicles via the Death Valley/Big Pine Road. From the Ubehebe Crater Road you must travel 44 miles of graded dirt to the dunes. From the town of Big Pine there are 28 miles of paved road and 21 miles of graded dirt to the dunes. The final 10 miles of both routes is the narrow South Eureka Road. During inclement weather, all access to Eureka Dunes can be closed or limited. No water, services or off road vehicles at the dunes. A similar drive to the Race Track (no dunes) is near there and well worth the visit in the early am or evening hours.
The dunes in Panament and Saline Valleys are aslo good. If you have a good 4x4, you can get to the Panament dunes from the Race Track.
Outside the southwest, the Oregon Dunes are fine and right on the coast. There aer also plenty of off road vehicles.
White Sands in New Mexico are easy to get to, unbelievably white sand, and a fairly small area. It isn't hard to see most of it in a day.