My wife is not fond of adventures based in caves or underground labyrinths. "Not another cave. Can we do something that's not in a cave?" While I love a good dungeon crawl, I feel her pain. The traditional dark, subterranean dungeon does get old after a while.
There are, however, very good reasons why the dungeon crawl is such a popular form of adventure. The rooms-and-corridors format of the typical dungeon keeps the choices of what to do next to a manageable level, and the fairly small scale - both spatially and temporally - allows for attention to detail. It takes only a few moments to describe the salient features of a room without leaving out anything important, so there's a sense of carefully exploring rather than glossing over large tracts of territory. What happens to them and what they find depends on their choices much more than on randomness. In the standard overland hex crawl, you can't possibly detail everything in every hex. Thus, either encounters are purely random, or else there's some keyed encounter somewhere in that vast hex, and the party experiences it simply by entering or passing through the hex, which smacks of DM fiat a lot more than it does of player agency. I know it's possible to have agency in a hex crawl, but it's always been a much trickier thing to me than allowing for agency in a dungeon crawl.
So, I'm planning something a little different, with a little more freedom of movement than an ordinary dungeon and the flavor of an outdoor adventure without the massive scale of a hex crawl: an outdoor "dungeon," if you will. I know this isn't new or original. It's been done before, but I don't know that it's ever really been looked at as its own thing.
Just as a traditional dungeon can take many forms - a cave, a ruined keep, a tomb, an abandoned mine, a tower - there are many possibilities for an outdoor dungeon too. The "dungeon" could be wooded valley with dense stands of trees and brush as "walls," game trails for "corridors" and clearings for "rooms." It could be a hedge maze, or a network of ravines and gullies. It could be a vast, sprawling garden, with well-tended paths and man-made brick or stone walls. It could be an abandoned town, with streets and alleys for corridors.
A major difference between the outdoor and the traditional dungeon is that explorers of the outdoor dungeon are much less bound to follow the designated floor plan. Climbing, flying, or levitating over the walls, and in some cases struggling or cutting a way through them, are possible. That's a feature, by the way, not a bug. That doesn't mean that such actions should be permitted without any cost or inconvenience, only that they shouldn't be outright barred.
It's entirely reasonable to say that a character hacking his way through the underbrush instead of sticking to the path is slowed to 1/3 normal movement and has double the normal chance of attracting wandering monsters. It's still an option that's open to him, one that he doesn't have at all in the traditional dungeon, and he can choose for himself whether or not the price is worth it. The air space over a hedge maze surrounding a wizard's tower might be patrolled by gargoyles or other flying monsters, and the party must weigh the convenience of flying over the maze against the hazard. A character levitating or climbing for a bird's-eye view of what lies ahead might reveal herself to monsters nearby, or even make herself a target for missile or flying attacks. Some parts of the "dungeon" might be obscured by forest foliage, mist, or other obstructions.
The outdoor dungeon might be a level of a larger dungeon which also includes traditional dungeon levels. A wooded valley (level 1) might lead to a cave (level 2-4) which in turn leads to the sprawling ruins of an open-air temple on a plateau above (level 5.)
In fact, I have such a hybrid dungeon in mind, which I hope to be drawing up and stocking over the next few weeks. (Well, knowing how I operate, it may take longer than that, but if it turns out really well, I might just slap it in a PDF and put it up for download.)