Bluetooth Only.
There is no option, there is no workaround. If you want to Apple Watch to play music, podcasts, or audiobooks, you need to pair Bluetooth headphones of Bluetooth speakers.
I've paired 4 different manufacture's Bluetooth headphones to my Apple Watch. Two of them 15 year old headphones, a pair of Sony Noise Cancelling headphones, and the Apple Airpods.
The most difficult thing is finding the headphone's instructions on how to put the headphones into "Disovery Mode". But once that is done, they pair up with Apple Watch very easily. And finding instructions on 15 year old headphones is not easy 😀