"By the by" is a commonly used phrase to casually introduce a new topic or provide an additional piece of information. However, there are numerous synonyms that can be used instead to express the same sentiment. These include "incidentally," "in passing," "by the way," "as an aside," "as a side note," "funnily enough," "interestingly," and "speaking of which." Each of these phrases can be leveraged to add an extra layer of meaning to a conversation while keeping the tone natural and relaxed. By utilizing these synonyms, language can be more diverse and engaging, making conversations more interesting and enjoyable.