So people have asked, "What's the best thing to run at a convention, that will fit into a 2 to 4 hour time slot, and not run over the time slot or leave the players with no end."
Simple Answer. One Page Dungeons. There is various contests on the web, called one page dungeon design contests, and hundreds of people send in dungeons crawls. Some with good stories, some with no stories. The best thing about these, is its on ONE page and you as the DM can just pick them up, print them out, read it in 10 minutes, and then start jotting down notes on the back.
Now you are saying, "well, how do you know it will work?" I used this method in many cons so far, including Gencon.
As a DM, you have to control the flow of the adventure. IF you see your players racing through the adventure, throw in some roleplay encounters, or some random rolled combat.
OK so you don't like on the fly. Then on the back of your sheet, add some notes for things you can use incase of this or that.
Always be ready.
Keep it original, and keep it old School!
Links to help:
-The Evil GM