I would probably run it by the self-serve car wash for a quick wash off, nothing major and maybe lightly vacuum the inside.
As an aside, people really don't pick up trash from inside their rental car? I would feel terrible dropping it back off with empty food wrappers/drinks/etc all over. I understand not taking it to the car wash, because that can be more of a liability (in case the car wash damaged it somehow). Not vacuuming? Fine. But to not pick up the trash?
I kinda equate it to hotel rooms. You wouldn't leave trash all over the room (on the night stand, desk, dresser, etc) for the maid to clean up. It all goes in the proper trashcans and sometimes even that trash gets taken out and put into the main hallway trashcan.