Joshua Tree might work. I like Anza Borrego more as a picturesque place. Early spring the wildflowers go into full bloom assuming we get rain. They are accessible by 2wd vehicle
http://www.vimeo.com/821715
We don't take the TM up the canyon though there's an RV park somewhere in town we've not tried. Upper Coyote Canyon requires skilled 4wd driving. There's hiking too so basically something for everyone.