Even obsessive-compulsive planners such as myself — you know, full stats for every NPC, meticulously-drawn maps, crafted handouts — will have to go off the page and run a session in a more impromptu manner (especially if those rascally players refuse to snag that adventure hook you dangled so temptingly in front of them and want to set off for the dark forest instead — you know, that dark forest you don’t even have a wandering monster table compiled for.)
More to the point, many GMs are readily comfortable coming to the table with their array of game books and little more than a list of objectives for the session.
How do you run a low-prep game?
I suggest making a shopping list, which is just a skeleton or outline of how the session might play out.
Follow the rules of three
Your shopping list comes in three sections, comprised of three items each. Brainstorm this before the group assembles or during a quick break when you try to imagine what lurks in the proverbial dark forest.
Your three sections are:
- Major encounters.
- Secondary encounters and story-pegged plot points.
- Nonplaying characters.
From there, jot down three elements you can use in game play, such as:
- Introductory combat, a warmup fight to get the PCs in the mood.
- A challenging mid-adventure fight, one designed to drain the PCs of their resources.
- Big Boss encounter, including minions and henchmen, usually in a compelling setting.
List the monsters or characters you want for each of these main encounters. You’ll be “running out of the book” as they say. Depending on the system, don’t hesitate to file the serial numbers off monsters to substitute for character class foes. (I especially recommend checking game designer Sean K. Reynolds’ blog  for his suggestions for repurposing Pathfinder monsters. I think one of his most inspired choices is to use the Xill to stand in for a 7th level monk.)
Also, don’t overlook compatible game system supplements. In an OGL d20 game, for example, the sample ordinary foes provided in the d20 Modern Roleplaying game and its Menace Manual can be ported over to a third edition D&D or Pathfinder game on the fly. (Just replace that gun with a longsword and you’re ready to go.)
Make a quick note about the terrain for each major encounter. Just enough descriptive flavor to paint a quick picture. A glade overgrown with thistles and thorns. A rickety bridge over a fast flowing stream. A bluff with a sheer cliff face drop-off.
Your secondary encounters are less structured. These are mainly notes about reward, say an item of treasure (maybe a magic item or treasure map), or story point needed to advance the party’s story (a clue or evidence of the boss monster’s plans), or an interesting trap, puzzle or skill challenge. Make notes about these three secondary encounters, and insert them into the session when it seems to make sense. Basically, you can lead each each of the major encounters with a secondary encounter.
Lastly, come up with three NPCs (or groups of NPCs). In a rough sense, devise one that will aid the party, one that will hinder the party and one for flavor, such as a goofy or eccentric personality. Again, if stats are need for these NPCs, use whatever monster from the rulebook makes sense. Interaction with the NPCs are great ways to foreshadow both secondary and major encounters.
So armed with a list of 9 things to do, and notations of the page numbers of relevant stats in your various gamebooks, you’re ready to go off the page and run an impromptu session.