The northern end of Death Valley NP entering from Big Pine, Ca.along the DV/Saline Valley Road in late Fall or Spring offers lots of loop options including the Eureka Dunes and the Racetrack areas.
You can connect to the DV/Saline Road via a more difficult and high altitude rock and dirt track from Independence, Ca. -the Mazurka Canyon Road to Badger Flat 9000' Then over the crest to Andrews Mountain Loop which connects with the DV/Saline Valley Road in Death Valley.
Find a copy of Mountain Biking Southern California's Best 100 Trails by Don Douglas for more details and other options.
You could link this route with the Bristlecone Pines Reserve just to the north of hwy 168 where you have Silver Canyon and Wyman Canyon for some great down hills after some serious high altitude travel in the dirt. You will have to carry all of your water needed as there is none in each of these areas.