This is in San Diego where there isn’t much public transportation. I wouldn’t take public transportation anyhow. Not much benefit unless its faster than cars.
Airport to Mission Valley by trolley works nicely — there’s no tram service to the airport, but you can take one of the rental car shuttles then walk a block to the Middletown or Washington stops. Yeah, it’s freeloading, but as what they don’t know doesn’t hurt them…
Did this when I was meeting someone who planned to rent a car later.