In an earlier post I talked about the importance of "getting to the dungeon" when writing action/adventure stories (the same concept can be in other stories but it takes some more abstract or metaphorical thinking in that case). Now I'll talk about the sort of things you can do once you get there.
First and foremost is the adventure and exploration aspect. This is where you build the environment for your Reader. Describe for them what the character (in this case, The Hero) is experiencing through his senses. What does he see in this dangerous place? What is he searching for? What kinds of sounds does he hear or the smells that he encounters? That sensory thing adds to the action that, even when included in non-dungeony scenes, is more staid than in dungeon scenes instead of being part of the action.
When writing the Journey to Chaos series, I can get a lot of mileage out of the environment that Eric or someone else is entering. I also like world building and showing off this new area and what it means in a bigger picture. Sometimes I get carried away and talk about the history of some minor thing that does nothing but clutter up the narrative.
There's challenges to find. The typical Indiana Jones dungeon would have some pit traps and giant rolling boulders. There can also be some puzzles and physical obstacles. Most exciting can be anything living in these areas. That's right; MONSTERS!
Monsters, whatever their form, are my favorite part of the dungeon. Designing them and pitting my hero against them is great fun whether I'm the player, the reader, or the writer. In fact, just last week I wrote about Eric fighting a golem in the draft of Journey To Chaos book 4 and it was the most fun part of that chapter. Not only that, but it led into the first scene with that arc's major villain.
That's the other thing about dungeons and the main narrative reason for them. Without a plot purpose, the hero is just going into a place to do cool things without a reason. Granted, this can certainly be a legitimate plot and a good one (who doesn't like a wandering blood knight?) but most heroes will have a reason for going into the dungeon. A lot of stuff can happen here: An existential hero's journey through the darkness and monsters, a bonding event between two or more characters, a driven pursuit to strike down a foe, rescue someone dear, or both. There are endless reasons to send your hero into this dangerous area known as a dungeon. It is a conflict creator.
This is why the trope In Media Res is employed and it is also the point my other post about whether Origin Episodes are necessary. Why not skip the foreplay and jump right into the action? You have the characters talk about that stuff while they're in the dungeon, between the traps and the monsters and the intra-character conflict and bonding. That way the information is present and available to the reader but the main thrust of the story is unrestricted.