Whimsy

A whim; a freak; a capricious notion, a fanciful or odd conceit.

A whim.

A whimsey.

Playful or fanciful behavior or ideas

an odd or fanciful or capricious idea; "the theatrical notion of disguise is associated with disaster in his stories"; "he had a whimsy about flying to the moon"; "whimsy can be humorous to someone with time to enjoy it"