Check out: at-will.omnivangelist.net/2010/10/gonzo-...
It works out pretty well.
The main thing that works with me and gamma world is not to try to create an adventure - since you want it to be zany but not forced. Don't get me wrong I like the modules but sometimes impromptu is the best bet.
If you want a quick weekend game. Start them off with some action: Suddenly an explosing and fight. It skips looking for the hook to a game and what are we doing... Since you want to guard a caravan - have the start be a raid on it where they are taken by THREAT
a) I try to look at mythology: the 4 horse men (death, famine, war, plague)
b) stretch it to something else with 4... 4 food groups: Grain - mutant horse, Protien - mutant bull, Fruits - mutant colorful zebra, Dairy - mutant cow.
c) add irony: riding lobotomized giant human/ape mounts.
Add a reason: 4 horse men make me think of the apolalypse - so something very bad will happen
a) reason: They are preparing for the worst where they believe all the potatoes in thier farms will suddenly vanish.
b) thier train of thought: Thinking mel gibson - apocalypto - They believe that they technology will help them so they are gathering it and sacrificies
c) lose potatoes... need a potato god: sacrifices to a large Mr. Potato Head Statue.
d) irony again: They yell "A-POTATO-COLLAPSE NOW!" while attacking the caravan.
Bonus points if a player plays a plastic plant potato.
edit* Follow up - if you want others to be the cause of the Great Potato Collapse. Have them be the "Black Eyed Peas" who are destroying the potatoes on moral standing.... or something
edit 2* maybe the Black Eyed Peas is an evil alien POD with 4 Peas inside of it. He has inhabitited the holy food pyramid that the horse people worship and his roots are boring through the ground killing the potatoes...
edit 3* the holy food pyramid could be a collapsed FDA building :P