Here's a cool thing you can do. Get some sort of simile, or a phrase that can be taken metaphorically, and use it in a way that the PCs believe it's just being used metaphorically, but then later on at a crucial point of the adventure they discover that it's actually literally true. Bonus points if their misinterpretation has caused some serious complication or difficulty that could have been avoided.

For example: Suspicious villagers say that the destruction of a nearby tower, previously inhabited by a friendly wizard, was "the work of the devil". The heroes assume this is poetic licence to describe some sort of tragedy or natural disaster, and head over to dig through the ruins in search of treasure. They find it really was the work of fiends from the nether realms, who the wizard had inadvertently summoned to this plane, and they're now building a base for incursions into the material world!

Or people say some villain "has the strength of ten men". Okay, so he's strong, the heroes think. No problem, we've defeated strong people before. Ah, but not ones literally ten times as strong as the strongest person in the party, who can lift boulder and throw them at them!

Further examples are left as an exercise. Think of a simile and turn it into an adventure!

Transcript

{Luke turns off his sword}
Palpatine: Shall the phantoms of my past still haunt me after my death?
Luke: If there's any justice, all the souls who suffered under your watch will.
GM: Luke, at your words you sense a presence in the Force.
Luke: Yoda? Is that you?
Voice: That little busybody? What a joke.
Luke: Who the hell is that?
Palpatine: My eternal nightmare; it speaks!
{beat, Vader}
Luke: You mean all this time you were... literally haunted?
Palpatine: The bulwarks... cracked; skeletal; but still standing.
Palpatine: Aha ha ha ha ha!