For example, in a similar fashion to how some NPCs run up to you as you're on your travels. A scenario could go like this: (in voice or in textual dialogue)
"Help, my family has fallen ill! Please, the town physician said they can only be cured by X item. I hear you can find it at the bottom of most lakes in Tyria, please help!" Or in caves, or in rivers, or on mountains and hills, or etc. The key point is, it's a separate kind of dynamic event that doesn't lock you into one zone; it would encourage exploration in order to get it done. And even at the locations that contain the item you seek, there could be all kinds of challenges waiting.
Edited by Pocario, 02 May 2012 - 11:28 PM.