3.5mm audio-jack adapters and splitters.
The Apple Lightning to 3.5mm Headphone Jack Adapter retails for S$14, though most authorised resellers were out of stock; I grabbed the last two from iStudio Paragon. The build feels fragile, with a thin, breakable cable.
The Moshi 3.5mm Audio Jack Splitter, at S$20.90, offers something most splitters lack, even the pricey Belkin one, letting one of the two connected earphones support a microphone. A splitter lets two people share music from a single device.
I bought the McKAL Airplane Audio Adapter from the new Challenger at Bugis Junction for S$6.90, or S$5.90 with member's price. Now I can finally use my own earphones for in-flight entertainment, and keep watching 10 to 15 minutes longer.