The best word to replace "whimsical" while maintaining both its denotative and connotative meanings is playful.
"Playful" carries a similar sense of lightheartedness and a sense of fun, which aligns well with the connotations of "whimsical." While the other options convey different nuances, "playful" captures the essence of a light, joyful, and possibly mischievous spirit implied in the original sentence.