something simple like this.https/www.amazon.com/TaoTronics-Bluetooth-Transmitter-Receiver-Wireless/dp/B01KO1JNCA/ref=sr_1_14?ie=UTF8&qid=1539450368&sr=8-14&keywords=bluetooth+audio+transmitter+and+receiver
You can connect the BT transmitter to either the zone 2 line level output on the back or the headphone jack on the front. You get different options on using it depending.
If you connect it to the headphone jack the receiver volume will affect the volume of the headphones and if it's turned up too high you will hear distortion. It may automatically cut of the speaker when the transmitter is plugged in or you may have to do that manually. Check the owners manual or try it with wired headphones. If you want the speakers and the BT headphones at the same time then use the zone 2 output. That will be fixed level and can be set to output a different source than the main zone if you want to. Some receivers won't output audio from digital sources to zone 2 so check the manual,