Drum roll please!
And it's... a room! The people there are taking a writing class and probably imagining things, but it's not quite the Room of Requirement, or Willy Wonka's chocolate factory, or Star Trek's holodeck like some of you folks suggested. The "Imagination Room" is in fact a room like any other... or is it?
Remember the prohibition episode of the Simpsons where Moe's Tavern turns into a speakeasy? He pushes a big lever that turns the whole thing into a pet store when the cops come. Perhaps the Imagination Room is like that. Maybe when I left the room, the big switch was flipped and it turned back into the "puppies and marshmallows" room.
We can always dream.