For design, I like the "sticky notes around a map" layout. Also, does the back of the page count? If so, a PC map, NPC sketch, or something similarly applicable could fit there.
For concept, however, I'll start with what not to do (or what's hard to overcome), based largely off my experience playing the AD&D product
Treasure Tales, which had 1 page for player setup and 1 page for DMs, and the (really bad)
Wonders of Lanhkmar (usually 2 pages), both a foray into brevity back in the day.
I had to use a lot of imagination to fit these into our ongoing campaign, and few were memorable.
I also took a look at
Sage Advice's link to people's homebrewed 1-page adventures, which almost all use the format of sticky notes or text blocks imposed over a 3-D map in the center.
1.
It all rises or falls on your primary story idea. You don't have room to embellish and explain.
2.
Brief adventures need to spark something in the imagination, not simply be a dungeon map. That's the main issue with the 1-page adventures linked above. Almost all of them are simply an abbreviated dungeon map. There's no "spark."
3. You need a twist more than you need a map.
So let's take the brief adventure "Whazzat in Der Swamp" from
Wonders. Merchant keeps getting attacked by crocodiles and hires party to guard him. Yep, in the swamp there's a dozen giant crocodiles that attack. Once you kill them, he pays you and it's over. Worst adventure idea ever. Yet, it's the concept put forth in most of the 1-page dungeons. Here's a dungeon, a monster or two, and a trap. You get treasure. The end.
Not all were beyond hope. One involved a Paladin who doesn't know he's guarding an evil artifact. The party is hired to convince him to abandon his post after he's given his oath. So we have a twist, the antagonist is a good guy. Another involved being hired to protect a house safe of valuables for one night against a robbery, allowing the party to setup traps. The encounters were the different tricks the rogues would use to get in, including paying a wizard to sneak one into the safe during other distractions. The entire encounter was text, no map of the house (just the primary room was described, leaving the DM to fill in the rest). But it's
Home Alone, which isn't something we get to do often in D&D. That's a
spark, allowing players to flex mental muscles in coming up with ways to thwart thieves.
Using this concept:
Richy Rich, designed for 4 players, level 4 (we can have a 2-story house floor plan, and the sticky note concept lets us put text anywhere we need)
Synopsis: Gotta have a brief setup, the core concept that makes us think "cool." So let's say the merchant Richy Rich is planning on leaving the city tomorrow and has withdrawn all his assets to his rental house in the city. His personal guards are to arrive on the morrow. He doesn't pay bribes to the local thieves guild and has just gotten word the guild plans on relieving him of his wealth during the night, before his guards can arrive. The city guard has been bribed and he's got no one to turn to. On short notice, he needs someone to thwart the robbery and sends his manservant Bob to the party. We also have our "spark," the aforementioned
Home Alone which allows us to set booby traps instead of the normal dungeon crawls.
Hooks: There's not much point if it's there's no impetus. Someone should already know Bob or Richy Rich, and a reward should be discussed here. Anything special should be included, like Bob having 50gp for the party to spend on supplies before night. We can probably fit this into the Synopsis if space is an issue.
The House: This is our map. We can sketch it, use free maps, whatever works. Ideally, players can get a copy since it's a defense adventure. If we're dealing with 1 page, maybe you print the house on the back? You really only need your notes of the thief attacks. Don't fill in special things like the crawlspace under the house unless the players ask. This is where you
can't put in cosmetics like a DC15 Perception to notice a creak in the floor indicating a hollow crawlspace likely below. We should discuss important features like lighting and have the surrounding streets in mind. We will put the locked safe on the ground floor. You can't account for all things players might ask (e.g. how do the windows open), but we might include it in the "attack" notes if they become relevant for reference.
Setup These are the "sticky notes" we see in the one-page dungeon design around the map. In this, we want a pool of our monsters, no stat blocks, from our MM (and maybe, maybe, other source books). I have absolutely no sense of balance in this, but we can say the DM has a pool of 16 bandits, 4 thugs, 2 scouts, a priest, led by a Bandit Captain.
Attack #1: At 11pm, the rogues send a wave of 4 human bandits and 1 half-elf thug to break open ground level windows/boards, covering one another with crossbows. Another identical wave uses a Portable Battering Ram on the door. They will persist in this way until losing 4 troops, at which point they regroup.
Attack #2: We'll give the party time to rest, but not yet. Around 1130pm, the guild fires bolts at the ground level (not aiming to hit anything and from awkward angles from surrounding buildings). This is meant to draw the party away from the team of 4 Halfling bandits that is sneaking in the crawlspace below to saw a hole under the pedestal. (our notes say the crawlspace is 4' high, giving disadvantage to any medium characters seeking to attack down here and, DM discretion, making certain weapons impossible to swing.)
We need the attacks to be special, so for Attack #4, the rogues might have contracted a wizard to cast a spell (invisibility or the like) to sneak in an Elf scout whose job it is to disable any traps or obstacles (+4 Disable).
In the end, maybe there's a bonus that advances the campaign, like the merchant tells all his friends or gives them a map or so on. Let's leave the door open for more than just loot.