I've always thought that a bar code on the back of the neck, discretely placed where it can be covered by a shirt collar, if desired, would make an excellent personal accessory. I've mentioned this to 99 on several occasions but, inasmuch as her senses of irony and humor are not as highly developed as my own, she continues to express her disapproval.
Bar Code Mill is an excellent site to visit for those who wish to design something special for their own necks.