iClever Bluetooth Headphones, $17 with code PXNV65EL

You could fill a swimming pool with sub-$20 Bluetooth earbuds, but not many of them boast 8mm drivers, let alone 10 hours of battery life. Even if you already own a pair you like, I recommend getting extra sets of headphones to keep in your gym bag or luggage, for when you inevitably forget your favorites.