At Gen Con a few years back, one of my friends played in a game where six, eight level Wizards, are pitted agains each other in a maze. First one to the centre wins. It's a lot of fun, and perfect for a fourt hour, convention style game block.
The Dungeon Crawl Classic would be of Goodman Games, throwback style dungeon crawl. I would condense the module, so as to fit into a four hour block.
It would be nice if we could provide prizes for players. My inital thoughts would be to see if we could get some prize support form some of the publishers around here, so we can provide prizes to attendees, as well as introduce them to companies they may not yet have heard of.