Still thinking about this; I have a complicated web of interconnections on my wall depicting the sorts of things I might try to generate with a hex-filler.
Suppose that the party does leave a particular town to explore an 'empty' hex ... not something that would be wilderness, exactly, but merely a twenty-mile zone without a substantial town. A bit rustic, but still relatively upon the beaten path.
This is something that is lacking in D&D. If you want to explore, the game is a bit worked so that you only have the two choices: civilized and uncivilized. And there isn't any point to 'exploring' anywhere civilized - it just has the same things that every other civilized place has: places to buy shit, places to sleep and places to leave from. If you're in an ordinary world, there will be a rumor sending you to somewhere uncivilized. There might be the suggestions that something uncivilized exists inside the apparently civil experience - such as catacombs beneath the city, or a dangerous unoccupied house or what have you.
But there isn't any point in wandering the streets of the local town "to see what there is to see." The DM will almost certainly be strapped for something to offer the wandering player that will mean anything within the confines of the game. There might be some kind of game/contest going on, or the opportunity for a brief encounter with lawlessness ... but this kind of thing gets so predictable that players will wander the streets just to get into a brawl. But heck, towns aren't just places for brawls.
I can't speak for the gentle reader, but when I go someplace new I'm interested in experiencing new things ... a unique local restaurant, an odd bookstore, some kind of funny-looking house or ragtag collection of persons who happen to live there. It is a kind of eye-nose-ear candy, but unfortunately while it keeps me interested if I venture forth to a strange burg, it isn't the kind of thing that can be experienced by a player sitting at a table with his character. Sensory exploration just doesn't cut it.
Sadly, when you ditch what you can experience with the senses, exploration loses its verve. So what I was saying about the party wandering into a relatively civilized, but unfamiliar hex - in reality, there would be interesting parts to be seen: pretty scans of vegetation, a local girl with a tight scarf around her hips tending sheep, the hint of honeysuckle, a winding road with blind corners, a quaint little bridge across a brook, muddy but interested to stare at ... and of course a few odd animals here and there to stumble across. You and I could easily pass a day wandering ten miles through any fairly populated area and feel engaged, but your players will never, ever feel that engagement sitting at your playing table. They can't experience the reality - they want elements of that day they can experience.
You're pressured, then, to produce something that will engage them - another game or contest, or another encounter - or else announce that nothing of real note happens that day, the word of which will land with a dull thud as players strike off their food and take note that they are one day closer to the wilderness, where things happen.
How do you get out of this corner? That's what I've been piecing together these last three posts.
1) Something might have potential for happening. If the event isn't going on right now, the party might receive word that something will happen here in the future - something the party is interested in. A festival might have some potential, but pretty much only if it increases the wealth, status or power of the player characters. An approaching army is a bit heavy handed. But if the party isn't going to be affected right now, and there's no promise that being here will cause the party to be affected in the future, the party might as well move on. In that case, what is the purpose of ever having been here?
2) The hex might offer some kind of service. We're basically back to shopping, but for services we can include lodging and damage repair. It follows that some places ought to offer better and more abundant supplies, be safer, heal players more quickly, be stacked with willing and generous spellcasters, etc. As well, information is an important commodity to be had, for rumors or hard facts about elsewhere. Thus, if nothing is happening, the hex might still be useful.
3) The hex might be an obstacle. Even if the party hasn't got plans to enter the hex, the imposing existence of a difficult to travel through hex might force the party to waste time travelling around it, or at least risking their supplies and lives when travelling through it. So not only should it be an obstacle to time, it should also be an obstacle to preservation of supplies, player wealth and obviously player survival - the latter being the one that is almost always the only one considered. There's lots of ways to obstruct survival, however, past merely encounters - but that was what I tried for when I proposed wilderness damage. You might try these posts on the subject here, here and here.
4) The hex might be an easement. Some hexes offer easier passage to other hexes, such as passes, fords, bridges, roads, ferries and I suppose even tunnels and interdimensional gates. Sometimes the vegetation is less dense, or offers level ground. Of course, habitations tend to grow along easements, but often the particular place is so obscure that is still remains too far from human populations to be settled.
5) The hex may offer an opportunity. In effect, some kind of resource can be found here that hasn't been exploited yet, or alternately the 'resource' is something like a dungeon or a tomb containing unusual articles that haven't been returned to civilization - or if we are still talking civilized, an opportunity for mining, industry or trade that no one else has happened to notice but which a party member undoubtedly will. Then it is up to the party member to decide to exploit it.
The list isn't exhaustive, but it is more or less where I'm at right now. Everything I've thought of can fit somewhat into any of the above lists; it helps to keep a wide perspective of what might be an 'obstacle' or an 'opportunity.' A bridge toll may not seem like much of an obstacle to the party, but it is nevertheless an obstacle that reduces the player's wealth; it's presence to an interloping party is obviously obstructive. However, and this is just as reasonable, a toll upon a bridge the party builds over a watercourse they've come to possess is just as much an opportunity to them as someone else's bridge is an obstacle. It depends upon the ownership and existence of the bridge in question.
So, while five kinds of filler may not seem like many, the potential numbers of filler may be considerable. It simply requires a starting generative abstract that can be added to with time and effort.