You'll still need to take notes in some way or another, I recommend a camera/cell phone but there's no reason why you can't write it down or simply take mental notes of what you see if you have a good memory.
O.k, enough set-up.
Go outside and take pictures (or notes) of anything that even remotely reminds you of the game you're running. So for example if you're running D&D, take pictures of anything Fantasy-related.
Here are the pictures I collected, taken from trucks, graffiti, store windows and a liquor store:
An Oriental Dragon.
Just by the title I'd say I'd include a Magic-User, but that's a demon.
So a fireball-spewing demon.
A Gnome, sure.
He's looking at the ground, so let's say he's Tracking.
A knight. No wait, two in a row, knights.
A skull of some important significance?
An undead? A representation of a poisoned bottle?
As this parodies Game of Thrones, I could have almost decided not to use it.
Too obvious. But this is a Gnome KING.
I'll be using that instead of anything GoT-related.
A raven. So much for avoiding GoT influences.
So now that I have my chaotic collection of inspirational material... I have to use it all!
This is what I came up with:
A Gnome King has witnessed a Demon Imp turn into a raven within the King's domain and fly off with the skull of his grandfather, stolen from its resting place. The Gnome King sends his best Ranger after the Demon Imp, fearing the blackest of magics and overall insulted at the desecration of his grandfather's tomb. The Ranger initially had a squad with him, but they were all killed during an ambush set up by two corrupted Human Knights (brothers) hired by the Imp. (The Imp mostly watched the fight from the branch of a tree while spouting orders at the Knights and the occasional Fireballs at the Gnomes!) The Knights are very competent and take full advantage of their riding skills when they have enough room. They did seem in a hurry though, which is why they left the Gnome Ranger for dead instead of taking the time to confirm their kill.
Following the tracks to a human city with a port, the Gnome Ranger learned that the knights left for a boat heading to the Asian continent of your setting of choice. Having no time to wait for back-up from other Gnomes, he sends a message to his King to keep him updated and hires the Player Characters to help him.
From there they all take a boat heading for the same place as the Imp and the two Knights. On the way they fight a Kraken, of course. Once they all reach the Asian continent, they face off against the Imp and the two Knights, as well as the evil mastermind behind it all, an Oriental Dragon!
(Quick reason off the top of my head, the grandfather of the Gnome King was part of a group of adventurers who once showed up in the same land the dragon lives in and thwarted his plans. It took the dragon many decades to find out the identities of all the adventurers involved and in the case of the Gnome, since the original adventurer was already dead, the dragon decided to settle for his closest living relative. The skull would have been used to curse the bloodline.)
Feel free to comment on this idea of preparing games and/or to share your own story based on those pics!