David Allen has a lot to teach you about running a good adventure.
If you don't know who that is, that may seem a little weird. If you do, then it probably sounds a lot weird. Allen is a productivity specialist best known for creating the "getting Things Done" methodology. I'm a big fan of his stuff, though I am not as disciplined a practitioner as I feel I should be, but that's neither here nor there.
One of the central tenets of Getting Things Done (GTD) is using well designed todo lists. There are a few elements to this but the one I want to bring into focus is well-designed list items. The idea is simple: when we usually make a list if things to do we tend to list the problem to solve ('pants'), or write something that's too big to be useful like 'Clean Garage'. Instead, you want to focus on the next action - you want to write down a task that you can explicitly *do* rather than something vague. The idea is that if cleaning the garage actually requires you do a lot of things - get trash bags, rent a dumpster or get your shop vac back from phil, for example - then "Clean Garage" is a lot less useful than "Call Phil about shop vac" or "Research where to rent a dumpster."
This comes back to planning adventures because that advice applies to the situations you create for your players. The next action is as important to keeping an adventure hanging together as it is to getting things done.
Nothing grinds a game to a halt like players having no idea what to do next. Gaming is full of habits and traditions (notably, dungeons and railroading) that persist because they provide an easy solution to the problem by limiting choices so severely that there's no paralysis. Things go the the next step (as conceived by the GM or the Adventure designer) because there's nowhere else for them to go.
At first blush this seems like there's already a solution to the problem that works fine with GTD, but the important thing is to remember WHO is acting here. Forcing the choice is a solution to the GM's problems, not the player's. The players are taking that path because they've got no other options, not because it solves anything that they want to solve.
So, if you've got a somewhat more freeform game, what can you do to get that same kind of certainty without forcing it on your players? That's where thinking about next actions starts becoming useful. The same kind of thinking that breaks down a task (like cleaning the garage) can break down a plot into the actual tasks that need to be accomplished, and thinking about things in those terms can help open up your sense of where a plot needs to go. It's uncommon for there to be only one task that must be done first - certainly some actions have dependancies, but there are usually multiple possible starting points.
So lets suppose that you, as a GM, take a look at one of your plots and break it down into an array of possible actions, what now? You don't want to just force your players to do them, or you might as well have skipped this whole step and just shoved them in a dungeon. No, the trick is that you need to make sure your players *understand* that these options exist. This might require something as blatant as discussing things outright with your players, or it might simply require a few hints. Either way, you want to make sure that whenever your players find themselves at a point in the game where they might wonder what to do next, make their options clear in very specific, actionable terms.
Similarly, if you find your players turning over a problem that seems to suggest obvious action to you, stop and consider that you may be skipping over a few next actions which are less obvious to your players than to you. Rephrase the problem in terms of the actions it requires and see if that shakes anything loose.
There's a lot more to GTD than just next actions, and while it's not all game applicable (I'm not sure what "Inbox Zero" is in game terms) you might be surprised how many of them are.
1 - It's a subtle distinctions, but dungeons can be slightly better or MUCH better than straight railroading. Some dungeons demand you go through pretty much every room, and in that case it's a railroad that gives you a little freedom in choosing the order you do things. But some are designed with the expectation that the characters will, in the pursuit of their goal or goals, not be entirely explored. These latter ones are rare, but they're pretty cool. The problem, of course, is that even if the objective of the game doesn't require clearing out the dungeon, players will often do so anyway because that's the only way to get treasure and XP, which rather defeats the whole purpose of it.