The best example I saw of GHI in use was a friend of mine who made a present for a roleplayer's "baby shower" event. Instead of getting something tangible with gold or crafting something with their chosen profession, my friend decided to use GHI to make something completely unique. The item was labeled as a gnomish baby-rocker, with a description that indicated it was a self-rocking cradle that played a lullaby for the baby as it rocked. The best part was that when the player clicked the item, it would play the sound file for Lament of the Highborne, making it a fully interactive little trinket for the "pregnant" player.
As far as unique items go, the only thing in my regular bags that by all rights shouldn't be there is Taretha's Necklace -- the one mentioned in Lord of the Clans that is retrievable via a horde quest. The last leg of the quest has you obtain the necklace because Thrall would like it back. This is all well and good, but when I looked at it from the eyes of my rogue, all I could think was that it was shiny, and pretty, and she would probably enjoy keeping it. So I did -- I never turned in the quest, and it is one of a few that are left unfinished in my quest log. But with the introduction of GHI, I could make other odds and ends that suited my character, like the journals of Arch Druid Fandral Staghelm pictured above. In addition, I could make copies of every readable book in game and carry them around with me as my very own mini library!
I wondered though, what with the particular ingeniousness of this addon, what sort of items other people would make for their characters. What would represent them, what would they consider something that their character just "had to have" whether it be for looks or just a fun trinket to play with. Would it have a sound file linked to it, or would it just be something to read? With GHI, the possibilities are nearly limitless. So if you guys could make one thing for your character to carry around, what would it be?