Maybe you're not in too bad of shape. You read over the game notes and find the map you drew. You said it was a fortification built into the side of a cliff. You trace over the map you drew them so you can give them the rough sketch for a player's map. A prop. It takes a few minutes to sketch out a dozen rooms. Hmm, not too bad, it's got some personality. You can see the story in the architecture, the shapes and design.
You have an hour before dinner and after that you have to take the wife out or she'll threaten harm on your tender areas. Better get a move on.
You calculated it will take three days travel to reach the dungeon. Two days will be spent going through a wild forest. You think about making a random encounter table, but decide to go with a few planned random encounters. The party will run into a few giant wolves as they enter the forest just to knock the rust off their dice. Later in the day they have a chance to spot a trail where it's obvious a patrol recently went though. It's an orc hunting party. The party can follow the tracks that led away from their destination. In this case the party has a good chance of gaining surprise. If an orc is kept alive he may tell the party his tribe has moved into the forest because of the noises. He can explain no further. Should the party continue on their way, later that night you can have the orcs attack the party's camp. Why waste a perfectly good orc hunting party. The next day in the forest they see sign of something very large has uprooted trees and left craters of footprints. A giant. At night tease the party with the ground vibrating. There'll be no sleep for them.
On the third day the party reaches the hills where the fortification is waiting. You're feeling generous and lower the difficulty level to find the place. Just to add some atmosphere there will be a bad rain storm making the terrain slick. Excellent. You figure to keep the dungeon as straight forward as possible at this point. You'll need to look through your monster manual to find an undead creature that would be a suitable challenge for the players to represent the restless soldiers. Shadows, they'll fit perfect. It will give the cleric something he can turn, something for the fighters to hack and the magic users to blast. In one section you decide to throw in a pack of Shadow Mastiffs. It adds some variety and will keep the players from getting too cocky. You populate the rooms then move onto who the boss. You thumb through the manual and reach the wraith. Hmm, the level drain worries you, but the party will just need to be more strategic than bull rushing every encounter. But, being a kind and generous master of the dungeon you make a note in the forest encounters to give one of the orcs a +1 bow with a few magical arrows so they can fight from a distance.
Now to put that chest of gold somewhere. Putting it in the wraith room is just too easy. You notice a mark on your map you didn't like and erased. That could be a good hiding place. The soldiers threw the bags of gold in holes. Anyone who can see in the dark or tosses a light down there will see them easily it will just be a matter of looking in there. You decide to put a few Shadows in there standing guard. Let the players figure out why they are standing guard in the crapper.
No dungeon would be finished without a trap. Let's take that original chest full of the gold and put it in the wraith room. Put a lock on it for the thief to feel useful. No needle trap in the lock that is too cliché. Maybe a fireball blows out the top when it's opened. Nah. Let's throw a teleport trap in there. The player gets a save, but if he fails he gets sent to a cell. The cell is at an undisclosed space. There are six dead bodies in the cell with the player. You could place the cell south of the dog pens, but maybe it could be used as another adventure seed. You note a few possibilities. Inside the chest are a handful of non-magical weapons to give it weight should the players decide to take it home without opening it. The teleport works once a week.
So there you have it. Let's break it down.
2 Random Encounters : If you need another encounter you can always throw in more orcs or that giant.
12 Rooms : A dozen room in the fort.
3 Monster Types : The monsters go with the theme and there is enough variety to keep the players interested.
1 Trap : Not only is it a trap, but a possible adventure continuation.
1 Twist : The treasure being in the crapper instead of the chest. Just slight enough to keep the players guessing.
And that's it. Within an hour you've come up with adventure that should keep your players challenged through the evening. It could easily stretch into a second evening and on to other adventures. The wife is calling. Dinner is done. You look over your quick dungeon. It's rough, but definitely doable. Tomorrow you'll have time before the game to smooth out some of the rough edges. The dungeon will be in part two.