Dirt/off-road routes from SoCal

So I'm in Riverside, CA. I'm going to be visiting Vegas in a couple weeks for the When We Were Young festival. I'm familiar with the drive (and the traffic) coming to/from SoCal and trying to do something a little bit different this time.

I want to utilize my 4runner and do as much of the drive off-road as I can. Are there any routes that follow alongside the 15 and/or aren't super far out of the way? I'm assuming we'll have to get through the pass via the highway and probably leave the paved road through somewhere near Barstow. Not necessarily looking for intense technical off-roading, dirt roads are fine. And I don't mind increasing the drive time if it means getting to see more nature and less 15 traffic.