Anza Borrego offers some great mild off-roading and beautiful nights. Highly recommended! There is a good book called "The Anza-Borrego Desert Region" by Lowell & Diana Lindsay (5th edition I think they are up too) that is a good book to have.
Took the van for some mild off-roading last March on the Truck Haven trail and over near the 17 Palms site:
Another great drive is out to Font's Point to the overlook, pretty amazing. 2WD may have a tough time with some of the sand washes, depending on where one camps or ventures. There are also some good camping spots for those who like RV sites:
On the other side of the Salton Sea a great day trip is to Niland to see "Slab City". That side of the Salton sea has some mild off-roading as well that can take you all the way to the Co River and if you want an RV camp look at http://www.glamisnorth.com/
I generally prefer to dry camp away from the crowds, so the Fish Creek site works. They have the Wind Caves you can check out near that area. I would also be open to the RV sites if most prefer that.
That Dec 7-9th works for me as well but doesn't for others.