For one-shots you can pick something up online or at a store. There are VAST amounts of one-shot ideas and fulls tories that would take little to adapt from any game to D&D.
Most times it is all about play styles and what one person wants versus the other person out of the game.
One reason I don't really like the RPGA is because playing a long lasting game with random people potentially each time would not be enjoyment for me, but more hassle than it is worth to figure out the other playes to form a coherent group with them and be able for each to find the weakness of the others and be able to compensate for them during play.
You don't have to write everything for your group for even one-shots.
Grab a loose idea, and do it on the fly. As a Dm this can very easily also spur ideas during play that you could not include in the on-the-fly game, and take for your long lasting game that can be built upon.
For me as a DM, the ideas never stop coming, and I probably take more notes during play about ideas, than either my players take about the game they are in, or I wrote for the session we are playing.
I save those up for times of DM block.
Best thing to do is always carry around some little pad/notebook for D&D purposes, so no matter where you are you can jot down a spur of the moment idea for something to flesh out or use as is later.