Can you use the credits to book two separate one ways, creating an open jaw?
Driving back and forth between northern and southern California to manage a r/t in and out of the same airport will eat up 2 of your days.
You could fly into OAK, spend the night in SF, drive down Highway 1 through Big Sur, spending the 2nd night in Santa Barbara, continue on to LA for a couple of days, and end your trip in San Diego, departing out of SAN.
If you can't book that way and have to do a r/t, consider dropping the car at SAN (in this example), and taking an intrastate flight back to OAK. The drive back up north on the Golden State Freeway is a slog.