Con Adventure Idea
For a convention (4 hour time slot):
PCs: Dual-class fighter characters in a coliseum.
This allows a straight fighter, fighter-thief, fighter-mage, or any other fighter-combo desired to spice up the game.
Background: The PCs are gladiators who are about to fight some fantastic monsters in an arena, but there is a twist. Under the coliseum is a "dungeon" of sorts. The rooms are either empty or contain items to make the arena fight more interesting.
The coliseum itself is warded by magic so that the gladiators and monsters cannot leave the arena (or the sky directly above for flying creatures). The stands directly in front of the king/ruler are further guarded. There are armed troops for the first many rows. Then there are troop commanders armed with magical weapons. Then there are powerful mages surrounding the king/ruler's viewbox.
First three hours: Exploring the dungeon below.
The PCs are allowed to recruit 4 of the monsters they find in the dungeons. Later, powerful mages will charm these creatures to fight on the side of the party. The PCs have to decide as soon as they meet the creature whether to recruit them or pass. The ones they pass will fight them later in the arena. When they have chosen 4, the rest of the creature doors are locked.
The PCs will also find other things in the dungeons: weapons, armor, other items of various uses. The party starts with nothing before the delve.
Final hour: Arena battle
The PCs and the creatures they have recruited have to battle the creatures they didn't recruit. Maybe get a second DM to run the PC creatures and I run the evil creatures? However it is done, get another DM (or two) to help run!