Audio cable
Although more and more cars support Bluetooth audio streaming or have a USB port for connecting a smartphone to a stereo, many do not -- especially not rental cars. But they have an auxiliary port, so carry around a standard 3mm audio cable. You'll also want it when watching a movie on a hotel or other older TV via a VGA cable.
If you travel with a companion frequently, consider getting an audio splitter so that you can watch the same movie from one tablet.
