We live in PHX and drive to SD a couple of times a year, and I completely disagree about seeing nothing, at least the first time you do it.
The exact route depends on where in the PHX area you will be. We live in Tempe in the East Valley and find the best route is the back way through the city of Maricopa to Route 85 in Gila Bend. Then pick up Route 8. This is also a good way to avoid the dreadful traffic on Route 10 during rush hour. After you've done it a couple of times the drive is a grind, but the first time is kinda fascinating. Be sure you notice the Space Age Motel in Gila Bend, which is a town that looks like The Land That Time Forgot. The motel is a hoot. Between there and Yuma there is Dateland, AZ, where you can stop and get a date milk shake. Why you would want to, I'll leave to you.
When you get to Yuma (good place for a potty break and to fill the tank with cheaper AZ gasoline), watch for the Colorado River. Once you cross it, you're in CA. A short distance in, you'll see some really pretty dunes, probably the only ones you'll see in either AZ or CA, since our deserts look quite different than the kind with dunes. You will go mile after mile past fields of hay and vegetables (remember that this is where much of the nation's veggies are raised). El Centro is not much of a town, but good if you need a break. A while later you'll see a mountain off in the distance. You'll see signs for Ocotillo, the town, and Plaster City. Sure enough, if you look off to the right, in the distance you'll see a giant gypsum factory. If you were to go there (no reason to, really), it'd look like plaster all over.
You're going up that mountain through some really crazy geology, including great rock formations and canyons. A feast for the eyes, at least the first time. You'll peak at about 4,000 feet and drive in the highlands for quite a while, through beautiful countryside. Some of my favorite non-ocean places in southern CA are up there -- Lakeside and Alpine among them. Just genuinely nice places with a great climate. You'll come down toward San Diego through El Cajon and La Mesa, with gorgeous homes draped on the side of the hills. Very California!
When you get to SD, say hello to Ocean Beach for me. And have a wonderful trip.