[Adventure Design] Need ideas for a 3 session adventure

[dreaded_beast]

After the next 2 sessions, I will be running an adventure for the following 3 sessions. I'm picking up from another adventure which the group only played once because I wasn't able to show up that session. This 3 session adventure will be separate from the current campaign. (Although I wouldn't mind it becoming a more permanent campaign).

Basically the plot is that a rogue cleric of Pelor, Simon, is on his way to the capital city of Karamekos to assassinate the Duke, Stefan Karamekos. The DM's world is a bit of a mix between Greyhawk and Mystara it seems.

I would rather DM in generic Greyhawk, but I want to keep the game consistant. I am toying with the idea of just using the basic plot, but changing the world to better suit my needs. I know the original DM wouldn't mind too much. I was thinking along the lines of maybe having the PCs enter a "portal" or just saying before the session starts that the world is different.

Anyways, does anyone have any ideas/plots/encounters/etc. that I could use for the above 3 session adventure?

 

[Squire James]

Since I just happened to be planning a 3-parter myself at the moment, you can steal and alter my idea if you want...

Session 1: Someone the PC's has reason to trust sends the PC's on a side-quest, involving a portal, offering a hefty reward. The PC's brave some dangers, find the portal, and go through it. The portal leads to an unknown location, and it should be clear that the PC's have just been betrayed.

Session 2: Wilderness-oriented PC's get to show their stuff a while, and urban adventurers get to show their cluelessness. No cities, no settlements, just animals and magical beasts for a while. Perhaps something like a dragon lair. At the end of the session, they abruptly find "civilization" in the form of a large group of slavers.

Session 3: The PC's fend off or escape the slavers, figure out where they are, return to the person who gave them the mission, and dish out some righteous revenge!