The piece of dialogue that would best help develop Casey's character as lighthearted and a bit playful is:
“Lots of reasons. There were nibbled food boxes, the scratching sounds in the wall—oh!” Casey grinned. “And I saw a mouse in the kitchen this morning.”
This choice illustrates her tendency to joke around and maintain a lighthearted tone, even when addressing a potentially serious issue. The addition of her grin further emphasizes her playful nature.