Apple's been offering free engraving on the cases of its AirPods and AirPods Pro since they've been around. You could enter some 15-22 symbols, depending on what the string contains - Ws and Ms are pretty wasteful, though going for the seemingly more economical all Is wouldn't get you past 21.
Anyway, the point is, you can now have an emoji engraved instead.
The list includes a plain old smiley face, the thumb of approval, alien, unicorn, and of course the poop emoji. Whether you'd like to have that last one on your AirPods is entirely up to you, but do consider one of the other 30 options. Sadly, you can't have an emoji and text and the same time.
By the way, we did try engraving an AirPods Pro case ourselves using the Beamo Laser engraver we happened to have at the office, and... let's just say you're better off sticking to Apple's service.