We've all been there. It's the night of your regular campaign and you gather around the table eager to roll some dice. Except, only two people have shown up because the others bailed last minute because of illness or work. The adventure you've worked on won't work even if you scale it down for two players, but fortunately you have a bunch of back-up games. But how do you prep when you're about to start?
Use Toys of the Sandbox
When you have no time to come up with a location, NPCs and an adventure hook then Toys of the Sandbox can help. Containing all the aforementioned information along with maps and encounters, having a copy of this tool with you at every game night will mean that you'll be prepared for a change in game. Right now Toys of the Sandbox only caters for fantasy systems, but a clever GM could re-skin it for other genres like sci-fi and steam punk.
Generate a random adventure
If you game with your laptop, then head over to the Donjon random adventure generator which gives you a smorgasbord of ideas for your game. With a framework containing things like story hooks, themes, villains and even red herrings, it'll take you about 5 minutes to sketch down your ideas and hey presto! you have an adventure. The site also has random dungeon generators and a bunch of cool generators for sci-fi games too. If you're running 4e then the good folks at Dungeons Master have even created a pre-made character library you can quickly pull from.
Run a co-operative story RPG
There's a tonne of pressure on the GM when coming up with an adventure, so why not alleviate some of that pressure by running a co-operative game. 3:16 Carnage Amongst the Stars is one of my personal favourites, as it requires about 4 minutes of prep time and the rest is up to the GM and the players to unravel the story as they go. Other such games are Polaris, Capes and While the World Ends, and the great thing is that they're relatively inexpensive.
Use Mythic GM emulator
I've yammered on about Mythic in the past when talking about solo RPGs, but one of the other uses for the engine is group play. The best thing about Mythic is that you need almost no prep: the GM and players come up with a premise and then the random dice rolls do the rest of the work for you. I recommend using rules-lite games with Mythic so you don't get bogged down with dice rolls and rules. Risus, Tunnels & Trolls and Swords & Wizardry work really well, I've found.