A Bullet Campaign is a format where you take a villain, determine their diabolical goal, then list out a few bullet points of steps they’ll take in executing their plan. It works great for high level campaigns, but works for adventures as well.
Here’s an example of an adventure written in about ten minutes, inspired by flipping through pages of monsters and magic items. It contains a backstory and motivation, then bullet points of events that may unfold.
“The Return of the Sky Elves” Adventure
Summary: Banished Sky Elves are staging a return with plans to enslave the local region.
Backstory: The Sky Elves are an evil, winged race living in the heights of a mountain. Generations ago the Sky Elves began plans to enslave the local region. Only the centaurs knew of the elves plan so they intervened to defeat them. A powerful centaur wizard forged an artifact known as the mage-ring to magically unmoor the Sky Elf mountain from its rock base, leaving a rock basin below which became the valley. The centaurs then for generations guarded the valley and the magic ring to prevent the Sky Elves from returning. Recently, a sky elf mage appeared to a local magistrate in a vision and struck a dark deal to pay mercenaries to recover the ring in exchange for power in their future, dark kingdom. These mercenaries have killed local farmers and used centaur-style arrows to frame the centaurs and set the stage for questing mercenaries to retrieve the ring.
- The mercenaries frame the Centaurs. The players are traveling in a rural location when they come across a girl crying over her dead father — killed needlessly as he farmed by distinctive red arrows. Investigation reveals these arrows are similar to those of a local Centaur tribe. His cabin has a map to the location of a mysterious artifact known as the mage-ring.
- Mercenaries spread rumors about the Centaurs. The local tavern has rumors of an evil centaur tribe bent on slaying all non-centaurs in the region. Some say the mage-ring which holds the key to defeating the centaurs.
- The farmer’s map reveals the dangerous path to the mage-ring. An impassable mountain range surrounds the hidden valley, but the farmer’s map shows the location of a hidden tunnel. Inside, the players will face encounters with 1) a catacomb slug, 2) a swarm of sharp-teeth cave imps, 3) water-elementals in a catacomb stream, and 4) a chained, lost relative of one of the players (but who is really a doppelganger).
- When the mage-ring is removed, the Sky Elves return as warring slavers. If the players arrive at the center of the valley they find a shrine with a statue (which later animates and fights the PCs) holding the ring. If the ring is removed from the statue, clouds will darken and a large mass appears overhead. The mage-rings power to keep the flying mountain at bay dissolves and an army of flying elves descend to attack the players. The centaurs, who were framed and really allies, not enemies, will appear to aid the fight.
For the above example, you can probably get by with just the first sentence of each bullet point. When you are originating the idea, you just need enough notes to trigger your memory, not a full description of the scenes you’ve created. Sketch a rough map if you want, make sure you have some stat blocks handy for the people and creatures, and you’ll be ready to run the adventure!