It has already been posted that you can keep the visual effect of certain back-worn quest-items if you don't turn in the quest.
What i just found out is that these effects are stackable. Afaik there are 2 types of "backpacks" that you can wear, from these quests (ally):
Extinguish the Fires - Quest - World of Warcraft
and
Freezing the Pipes - Quest - World of Warcraft
Here are some pics:
Might not be everyone's cup of tea but i find the effect rather cool