For my own game. I was fine for ideas on the overland portion. But fell short for the dungeon crawl.
* brief backstory- 3 young adventurers-in-the-making tell PCs of a dangerous, abandoned mine said to be brimming with precious metals & gems within the walls...except a strange race of humanoids (among other creatures) make it difficult to just descend & grab. The 3 will take the PCs to the general area if they take them on as hierlings.
1) Within the mine there is a group of young spelunkers that breached the mine via another cave system. They are hopelessly lost and need to be escorted to safety. These are effectively 1st level Expert NPCs and therefore exceedingly vulnerable to natural hazards or enemy attacks.
2) There is a dispute over ownership of the mines. The deed paperwork lies deep inside in a vault. The rival mining companies have dispatched hired crews to retrieve the deeds. The PCs must arbitrate an in situ dispute between the companies before things turn into a bloodbath. The crews don't realize it but the entirety of the mine is a colony of molemen/grimlocks. A loud bloodbath between the rivals will bring the entire colony down upon all of them (and you), leading to ultimate doom. Think of the movie The Descent. These creatures are the perfect predators (Spider Climb, Blindsight, the works) for this environment and there are thousands of them skulking the mine/cave system.
3) The mineral deposits are the result of a localized extinction-level-event meteorite collision ten thousand years ago. However, while within the mine, it becomes clear to the PCs that the meteorite isn't an extraterrestrial rock. Its actually a Primordial or a powerful entity banished from the Far Realm. The original owners of the mine found this out just before they violated the mineral boundary layer that serves as the creatures tomb. They shut down the mine and worked feverishly to erase the history of its existence so as to save the world from apocalypse. If this creature awakens, it will be end times. Uncovering the truth of the sudden shutdown, keeping the Primordial/Far Realm entity asleep, and convincing those staked to its ownership will separate success from failure.
4) Catastrophic earth movement opens up a crevasse. The PCs fall deep, deep within the earth and must find a way back to the mines. The natural complex they are trapped in is without light, scorching hot or freezing cold and toxic gas threatens them at every turn.
5) A long lost golem or clockwork excavation technology is discovered by the PCs. If they can figure out how to "turn it on", they will revolutionize the current era's mining infrastructure.
There is 5 for you.