Not so for AD&D and many of the other games which followed. I was going to include Runequest in this but I would argue that Runequest was really the first non-D&D sandpit with the map of the maps on p108 and 109 as the implied sandpit and wilderness setting.
Anyway I have never in 30 years of gaming found adventure construction easy. I used to see this as my own short coming but in retrospect some of that was perfectionism and the other part was simple fact.
The structures provided in many games for adventure construction were really lacking. I see many of the contributions of the Old school movement being a codification of sandpit design. Sure nu-school is doing something similar too. Most notably the contributions of the story games crowd like Cam Banks, Jeremy Keller, Luke Crane and many others.
So this brings me around to the systems which set the GM up for success. Leverage, Mouse Guard, Space Princes. they all include simple systems to put your game session on track and more or less keep it there.
It seems fairly simple to me now, so I'm going to break it down. I want to come out and state for the record that I'm a strong proponent of player agency however this does not mean the game should allow all actions to succeed so the player can simply play out a power fantasy. Quite the opposite actually - I think the games systems should respond in a very real way to the more fantastical wishes of the players and impose a bit or reality on those more fantastical wishes.
Think of your own life. Despite the best laid plans and preparation, things don't always work out the way we imagine. In a drama this is the very essence of the story. Action-reaction. Desire, impetus, consequence and the reverberating effects on the 'system': The, girlfriend, the family, the workplace, the bar, the vampire clan, the castle, the Imperial senate.
The point being the character needs to be acted on in the story to create drama. The player may not all ways get what they want. They can respond with any idea they have. That to me is player agency. To make choices and to impose those choices on the world - however the 'world' to has agency and is not passive.
With that out of the way how do I plan for a game. Well I dont really even need to plan it per say. I do all of this on the fly quite a bit. Mouse Guard is pure genius because it builds this into the game.
You need a structure so lets not reinvent the wheel [heh]. You need to decide on a 3 or 5 scene session. Do a three scene session as its just less work. Often what will happen is additional scenes will organically/spontaneously emerge during play. Not fight this, just let it happen. As you see part of the session design will manages this too. Its critically important to let this happen as this a) gives the players 'agency" as we talked about above and b) levels the playing field - when the players come to the table they dont know the direction of play and as GM there are times you too should have to manage with this aspect of role playing gaming too.
We need to pick what our scenes are. I mean the location for the scene at this stage. To do this, think of great stories and great locations you remember from them. The cave behind the waterfall, the submarine base on the ocean floor, the luxury penthouse with expansive glass vistas. Brain storm a few ideas. jot down at lest five and pick the best three. You will start to see a patten in session design as everything will be done in threes and fives.
We don't have to be concerned with dialogue but we do have to be concerned with the forward momentum of the session. I call this 'Impetus'. In a good game system the characters come preloaded with some existing Impetus in the form of setting related hooks, goals or relationships. For simplicities sake we are going to use just three forms of Impetus. You can think of more or refinements if you like but these will give you a session of play every time.
- The Get (this includes find and explore and all those permutations) PC's need to get something or someone, for something or someone - Why? Why? Why?
- The Stop- PC's need to stop something or someone or maybe the PC's need to be stopped - Why? Why? Why?
- The Give - Someone or something the PC's know will be given something or someone Why? Why? Why?
As you can see to make this work you need some things and I well build game engine will provide these for you. You may need location maps so get them sorted or be prepared to wing it.
You will also need to identify objects - people and things which you can being to the table when the mechanics of the game demand them. Most 'monster' books will give you 'bads' to knock down. Good players will give you relevant NPC/GMC's to link into the story. If not invent them on the fly. How? every time the players want something they have to interact with people. Make those people relevant. To assist in this have name lists ready to use for these instant somebodies or prepare them in advance.
As your going make sure you can answer for this story "Who, What, Where, When, Why and How"
You now only need to add the PC's to the first scene and push them with impetus. If things slow down throw a 'contest' at them. This can be the proverbial smoking gun or some more subtle test but make it relevant to the players right now.
This is my off the cuff writing of how I do it. I'll give it a bit more thought over the weeks to come and try and codify it some more.