I'm running some old-school Red Box D&D tonight. Last session, two characters traveled out to Fort Chesterfield, where Lord Chesterfield's mercenaries were being mauled in the night by a monster. These two intrepid heroes went into a cave and found an ogre and a goblin, killed them both, and found a tunnel leading further down into the earth.
I haven't really had a lot of time to plan stuff. I was going to use some Purple Worm God ideas, and whatever else comes to me.
But hey, you're here reading this. What do YOU think should be waiting down that tunnel?