Your doing a one shot and letting the players make the characters?
In the games I run for cons, the best part is the pregenerated PCs
each should have a reason to be on the quest, and existing relationships with the others. I usually try for 1/3 of the quest to be intrigue or infighting in the party, 1/3 puzzels & RP and 1/3 combat.
The last one shot I ran was a quest for the empresses' headdress. It needed to be back in time for the wedding of the Pasha. One pc was the bride-to-be in disguise, and the only PC who actually wanted the headdress returned was the vizer. (the only evil character) retrieveing the headdress wasn't evil, it was just a way to increase his standing with the Pasha. The bride-to-be ran off with the barbarian hero who had been blackmailed into the quest...
Who cares if the party dissolves, if they have lots of internal conflict that makes the mission harder, its only one night. You can't do a regular campaign where the characters are rivals, it would fall apart to easily. I also go for clandestine or courtly love affairs, long lost realtives, monsters pertending to be people ... Each charater should start off with secrets and make the finding out of each others motivations play a large role in the game.
[end rant]
(with a nod to WS)
Players ship is destroyed in a storm and they are washed up on a deserted coast (not actually an island)* Thier ship was wrecked by a mad wizard/ or psion, and his demonic henchman. One of the players is his former apperentice, another's lost sister was raised by the wizard and the sister falls maddly in love with him. A third player is a magically created being(elan) that was created by the mad wizard. Another was a master thief who was just pretending to be a sailor. With enough players use two ships that were destroyed fighting each other and the surrviors have to help each other - but how much? is betrayal inevitable? Make the wizards insanity reasonable, like paranoia, not just random cackling. He will have numerous guards and traps, all with solutions that the lost sister can use. Remeber that magic traps only cost 50gpxcaster levelxspell level and require craft wonderous item feat. (for 1 shot traps)
* or set it on the ninja pirate island of dinotopia

*