Where can I return the car I rented in San Diego on a Sunday if you are closed, as I will be taking a cruise out of San Pedro on a Sunday. Is there a shuttle bus from your office to the port?

Anthony has it right. That is the best way to get it done. Uber to the terminal if the hotel doesn't have a shuttle.

You can drop the car in the Enterprise lot (west of the Crowne Plaza Hotel entrance is off of 7th street) . Take the keys to the lobby of the Crowne Plaza hotel next door and there is a drop box there. No shuttle for Enterprise that day, but hotel may have something or might Uber to the cruise terminal.