For a one off, I like to collect monster, treasure and location ideas from the players at the table, jot down a few more specific ideas, pieces of colour and stats while they make their characters, then wing most of it. If you get keen you probably have time to write a front in that time, but I don't usually bother: I'm aiming for a simple, action-packed game that doesn't have any ambitions beyond the 2-4 hours available.
If any of that sounds helpful, I can expand.