I have a bunch of whimsical items that could potentially serve as plot-hooks:

The Wig of Persecution

The person wearing it suffers a -1 penalty to AC on account of being persecuted. Otherwise, it looks like a proper barrister's wig; possibly to make the item's name more confusing.

The Book of Canon

A finely crafted book about local legends and fandom disputes. Once a day the book can be transformed into a cannon (treat it as fireball spell cast by a 6th-level spellcaster).

Belt of lint pockets

It's a belt with many pockets which can be used like normal pockets. However, you can always find a pinch of lint inside of those. Lint can be used as a substitute for worthless of cheap (6 copper maximum) spell components.

Lute of invisibility

Sustains the effect of invisibility spell as cast by a 4th level spellcaster for as long as you play the lute. Everyone can still hear you loud and clear.

Axe of Grindness

It looks dull and in a dire need of grinding to everyone looking at it. Onlookers inspecting it closely (i.e. perform either an evaluate, UMD, search or spot check) feels compelled to grind it. That's it. The DM has always been such a dickish smartass. You should punch him.

EDIT: Just as a bit of background, I rarely GM humorous/gag scenarios, but even a serious campaign can use a bit of levity. One of my players was actually very fond of the Belt of Lint Pockets because it was so incredibly convenient.