The term "tops off" usually implies taking away the uppermost part of something. Therefore, the antonyms that come to mind are "addition" or "insertion." For instance, if someone says, "I will top off the soup with some sour cream," the antonym could be "add some heat to the soup." On the other hand, if someone decides to "top off" a drink, the antonym could be to "insert a straw." However, if "tops off" refers to completing or concluding a task, antonyms could be opening, starting or beginning. Thus, depending on the context, the antonyms for "tops off" can vary.