Puzzles and actual adjectives to progress to next floors, besides mindlessly killing mobs.

Perfect example is how they did the Haunted House event.

Finding clues to opening chests.
Finding/Standing in circles to unlock the path forward.
Lighting lanterns. (No, not all at the same time or with some time restriction xD)

Things of this nature. Since you'll most likely be 60 going into the new floors, killing mobs all the time isn't needed as much going into floors 51-200.