Monster Manual Plot Hooks

There is not a lot of description to the monster in the Monster Manual, but enough for a number of campaigns.

I will start noting down some ideas for low level characters.

Elves (MM1: p.106, Encounter Level 2)
The player characters have to cross a wood and are ambushed by elves that got enough of armed humans (etc.) crossing their territory. After all the civil war in the human lands creates ambiguity for them and they don't really see a difference between the factions. All are cruel, armed and therefore evil.

Fell Taint (MM2: p.104, Encounter Level 1)
The player characters are hired by an old astronomer (a starlock?) to accompany him to a mountain observatory. Once he starts his work there Fell Taint start to appear. What looks like an act of evil is actually an accident.
Variant: They are hired by 2 astronomers, a young brother and sister couple. Both of the two plans to see when the stars are right(TM). But they don't know about the fact that the other one turned starlock as well. When the PCs think they have figured out who the evil one is, the other one turns even more dangerous and lets his/her mask fall.
You get much more info and stats for Fell Taints in this article from Dragon Magazine 376.

Gnome (MM1: p.134, Encounter Level 3)
RRRoar! They are interesting because of their connection to the Feywild.

Halfling (MM1: p.152, 2x Encounter Level 2)
Every cliche of gypsies that travel around and have drakes (MM1: p.90) instead of chicken and horses. Offering to sell their kids to heroes for a few silver coins?

Homunculus (MM1: p. 156, MM2: p.143)
These creations make good encounters because they can be in the service of nearly every lowlevel mage or sage. Especially the stationary ones from MM2 seem to be interesting.

Orc (MM1: P.203)
An orc seems to be an amazing thread to a first level party. He is supposed to be just an orc but he can appear as something exotic and dangerous in a human countryside environment. I imagine him as leader of human mercenaries in a rural human civil warish area. (Could be that his stats are not fun to fight though, because of high AC for example. I have to check that out.) Maybe it is two orc brothers leading the bandits.
This mercenaries are probably the only hope for a human village, that otherwise gets run over by the enemy forces, but they are barbaric and cruel and earlier or later would get lynched if they were not so strong. And if they get killed some more orks will want to have revenge. Could be that the main army of the orcs arrives and they find their (littel) brothers dead.

Skeleton (MM1: p.234, MM2: P.183)
Undead cavalary (Skeletal Steed, MM2: P.183, Lvl. 3) is for sure some of the most impressive enemies low level characters can face. They seem to be especially to handy, to ride in a small village and scare the shit out of the denizens.
Maybe spice them up with a few Decrepit skeleton minions (MM1: P.234) and if you want to be really mean, a Blazing Skeleton (ibd.).

Wererat (MM1: p.180, Encounter Level 4)
This the perfect low level city gang encounter - spiced up by the transformation of some bandits at a crucial moment.

Here you find part 2 of the Monster Manual Plot Hooks.