The term "upset apple cart" refers to causing disruption or chaos in a situation. However, there are several antonyms that can be used to convey the opposite meaning. One such antonym is "harmony," which suggests the presence of order and peacefulness. Another antonym is "stability," which emphasizes the idea of a firm and secure foundation. "Smooth sailing" is another antonym that suggests the absence of obstacles and challenges. Similarly, "organized" and "tidy" are antonyms that imply a well-structured and efficient scenario. Thus, depending on the context, there are multiple antonyms that can be used to convey the opposite of "upset apple cart.