Every village, town and city has a large expansive sewer systems with tunnels larger than the streets. The theives guild is constantly running around through them while a group of wererats led by an evil cleric live in them.
The standard dungeon of randomly placed rooms filled with randomly placed monsters (some of which must have been born in the room to get through the door) that have random treasure. Of course, some rooms are just protected with random traps instead. No matter how much noise is created in one room, it can't be heard in the others.
A medusa in a 1st level dungeon to make characters wish they'd remembered to buy a mirror. For that matter, standard packages in the "adventuring store" of wolves bane, holy water, holy symbol, and a mirror.