Apple is expanding the free engraving options on AirPods cases to include emoji. The selection is fairly limited, ranging from a few faces (such as my personal favorite, the grimace) through to heart and star symbols. You can also have a ghost, robot, alien, skull, hand gesture (but no middle finger) or an animal emoji adorning your AirPods case.
Since last March, Apple has offered text engraving on the cases. You can make your selection when you go to add one to your bag. Just be sure to think carefully what it might say about you if a friend catches a glimpse of a poop symbol on your case, though.