In this post-3.5 mm world, Bluetooth is the most universal option. But a lot of older electronics still don't support wireless audio. Welp, now there's a dongle for that.
The AirFly dongle (from $34.99 on Amazon) plugs into any 3.5 mm headphone jack (mini jack) and provides a battery-powered Bluetooth receiver for that audio output source. So it's not something you would plug in behind the TV or leave somewhere long-term, but if you find yourself in a place with an aux port and you only have a pair of AirPods, you'll want to have this in your laptop bag.
You just charge the dongle with the included micro USB cable, then plug in the included two-headed headphone cable. One end goes into the dongle, then the other end goes into the 3.5 mm line out port.
Get the dongle in pairing mode if this is your first time using it with your headphones, then do the same for your headphones themselves. AirFly will then automatically connect to the headphones as soon as it sees them in pairing mode.
But know this up front: the classic AirFly dongle is a Bluetooth transmitter, not a receiver or a transceiver. This means it only works with audio jacks that are outputting audio, and the connection only goes one way: from the output source to your Bluetooth headphones. So you can't pair your phone with AirFly Classic to turn a speaker with an aux jack into a Bluetooth speaker.
If you want something like that, you'll be better off with the AirFly Pro ($54.99 on Amazon). It's a transceiver, so it can provide a Bluetooth connection that works as either an input or output. It also allows up to two sets of headphones to be paired with it, meaning you and a friend can share an audio output while wearing two different pairs of earbuds.
Even AirFly's cheapest options come with high-quality audio codecs like aptX, Bluetooth 4.1+ support, and 8-hour battery life. So unless you're on an intercontinental flight, it'll last the whole way and you'll have crisp, clear sound.