Outdoor Dungeons

[Seharvepernfan]

The OP got what he wanted, but I have stuff to say.

If the "forest dungeon" you are building is just a small part of the forest, like a few hundred yards/meters to a side, then you can design it like an actual dungeon. It could be an area with especially thick, interconnected trees, where the underbrush is somehow dangerous, and canopy is either very hard to get to, or is dangerous as well.

So, the "rooms" of this dungeon are clearings with no especially dangerous underbrush. The "walls" are the thick trees (thick both in girth and in how close together they are) or the dangerous underbrush. The "hallways" can be paths between the trees, root tunnels, tree branches, or holes in the trees. The "ceiling" is just the increasingly thick canopy, which can be traversed by climbing or flying.

The underbrush can be dangerous for a number of reasons. It might be especially tough and resistant to fire (as some trees/plants are), as well as barbed and poisonous to the touch. The players can go through it, with effort and/or expended resources, but they probably won't be able to go through all the "walls" like this. This should be made clear to the players.

The canopy can either be just a pain in the ass via simple logistics (hard climb checks, too much distance between the branches of separate trees to jump, etc), or it can be dangerous due to aerial enemies (maybe flying predators that for some reason can't or won't attack the players on the ground, or shimmerling swarms, flowers that release poison dust if somebody gets too close, etc.). Maybe dryads in the trees take offense to anybody climbing them, and attack the players unless defeated or persuaded not to.

Now, your players are going to think that you're just forcing them into playing through a "dungeon" in the normal way. So, only this small part of the forest should be like this, and there should be a reason for it that is known to the players (like, this area is home to fey lords/druids/elves who built it this way to keep people out). Also, if the players actually are capable of going through the "walls" or traversing through the "ceiling", you should let them - they gave their characters the right abilities or items to make this possible, so they should reap the benefits.

The "rooms" could contain land-bound animals/beasts/fey/plants/whatever, and treasure could be found on the corpses of those who came earlier, or in secret hidey holes and camouflaged caches left by the fey/druids/elves.

Alternatively, the "dungeon" could be the bottom of a chasm or valley in a forested area, so that the "walls" are the cliffs that lead out. You could also make a grid/hex map where each square/hex is a certain distance, and then you could populate each one with specific enemies/events/challenges/places/etc.