Sounds like you're up for some adventure:

Rent car. Drive toward Downtown LA and stop at Tommy's Original World Famous Hamburgers...2575 Beverly Blvd, LA 90057. They are open 24/7. The have been in business there since 1946. The earliest I can remember eating there was in 1961. Sloppy, mess burgers or hot dogs, soda.

Next head for Dana Point in South Orange county. danawharf.com Choose 1/2 day fishing (starts at 6:30am, should be there 45 minutes early) or less time and money, go for Whale/Dolphin watching. It is not prime whale season, but they are still getting sightings...see website. Have a late lunch.

From there drive to SAN, return car and shuttle to Airport...exhausted.

My car rentals from LAX to SD county are a bit more as I drop them off in Escondido, where I live.