My DnD Adventure Design Guidelines...

I've been a DM off and on (mostly on) on a regular basis for over twenty years now and there's a few lessons I've learned over the years about making a good adventure. So I thought I'd share them.

One: Have a complete story and make it short.
Nothing kills an adventure better than it dragging on too long or going on one that has no real point. This may vary depending on your particular group and how often you play, of course.

Have a Big Problem that the PCs need to solve.

Have a Big Bad Evil Guy (BBEG) who is causing the Big Problem. BBEG rarely gets any screen time since everything revolves around the PCs so in order to make him/her/it memorable make sure they know a lot about the BBEG before they ever meet them. And make sure there is plenty of talk about the BBEG and plenty of evidence of just what bastards BBEG and his ruffians are.

Think of Star Wars for a second. Think of, say, Luke and Han and Chewi and R2D2 and C3PO. Then Think of Darth Vader. How much time did they spend meeting with one another in A New Hope?

None.

Darth killed Obi-Wan (an NPC) as they were making their getaway. Leia (possibly also an NPC in that film... at least until she was rescued, that is) got some more face time with Darth and was the only one to see, say, Grand Moff Tarkin. But that was it. The main enemy for the group was the organization - the Storm Troopers of the Empire. The killing of Obi-Wan setup Darth as the villain for Luke in the next film. And of course Darth was the one who killed Luke's father, according to Obi-Wan. More tangibly, though, was that the Stormtroopers killed Aunt Beru and Uncle Owen.

BBEG needs an organization of underlings of various importance. This organization should have a recognizable theme of some sort to give it an overall consistency. While not every foe the PCs fight needs to fit this theme it's a good idea if there is a theme representative of some sort present in each encounter. It's also a good idea to have some variety here. You can only have so many fights with, say, the same half a dozen stormtroopers before the fights get sort of routine and predictable. Sharpshooter Storm Troopers (oxymoron?), heavily armored blaster rifle-toting Storm Troopers, and Orange Shoulder Pad Leader Storm Troopers and the like might break things up a bit.

Setting - You need one. I'm a big proponent of creating your own from-scratch setting for each new campaign. Mainly because I enjoy making them. This can be accomplished with a few basic elements if you need to create one in a hurry. You need a Map, a List of Names, Races of the World, Common/Uncommon Monsters You'd Like the PCs to Fight, Nations of the World. But most of all, you should make your setting small. Why small? The adventure is essentially short. Travel time may be limited. And even if the PCs change the very course of history for their deeds they can do so for one small section of the world and still have it be very significant, right? The rest can be "Flavor Country" which lies outside the range of the adventure.

Again, with Star Wars there was a whole universe out there but really all that the DM fleshed out was Tattoine and the Death Star with a minor nod to Alderaan. And of that he only developed a small region Luke lived by, Mos Eisley, The Landing Bay of the Death Star, the Prison Level, and the Outer Surface for the end fight. So, see, you don't need a ton to pull it off.

So. Draw up a map. Slap some names on it. Decide where the races primarily live. Divide the map up into nations by imagining how the race would fight over time and pick an interesting point to stop at. Here, by doing so, you already have a basic history sketched out - all from a map! Your monsters list will allow you to plan in some local color into the adventure. Pic one locale and set your adventure there. Detail the area as needed.

Player Handouts. You did all this work making up a locale so now type it up in a word document and break it down into a few basics. Include the Region the characters will adventure in, a brief overview of recent history, and include any other fact you expect them to know (but that they probably won't memorize in-game).

Work with Players on Making characters and help them fit into the setting somewhere. Discuss their character's back story and what kind of adventure the player wants to go through. Include connections to the character's back story within the BBEG's organization if possible.

Choice and Adventure Design.
A more recent discovery in terms of this particular solution to the always problematic task of balancing player choice with DM prep time and/or Railroading. My most recent solution to this is the following: Tie player advancement (XP) to completing objectives that fit with their character's aims. Make at least one more option of storylines to follow than you have PCs. Make each one of these storylines worth a varying amount of XP for each character.

Make these objectives transparent from the very beginning so they have a definite motivation for role-playing why the group should tackle storyline X next. Make it so that whatever objective they choose last will no longer be an option by the time they get to it. They can only do so much. Whatever objective they do not choose, the BBEG wins there. Then offer up several new objectives once those first few choices have been completed. This structure keeps you from having to prepare way too much ahead of time - you can develop it in chunks while still offering the players some choice.

There's many ways to tweak this sort of structure. You could have three options on the table at any one time - other options opening up as quests are completed. Maybe some options disappear as a result too. Maybe the players pick option A. After they complete option A they discover that Option C is no longer an option but B still is. D is now an option - stemming from their sucess with A and E is an option, stemming from their failure to tackle C in time.

Make Important Underlings to the BBEG. These important underlings serve to direct the forces that the PC go up against and act as mini-bosses that makes the players feel like they are making progress along the way. It also increases the face-to-face villain time in the adventure.

So, plan out a map of all your possible encounters as the adventure progresses. Fill these in enemies to fight keeping in mind that you may need to add more enemies depending on whether or not the PCs will level up before they run through the encounter. Plan out where your Important Underlings will be and make sure to build in evidence of their evilness or toughness in the encounters that occur before their fight with the PCs.

But... what about Dungeons? you may ask.

I usually don't use them. In my campaigns there are villains to pursue but for the most part they live and exist in fairly normal fantasy settings. A small base or fortress is probably as close as I get to a dungeon crawl of any sort.

And that's the basics of my typical adventure design. I hope this gave you some ideas to try out with your next campaign.