The phrase "off on" can be replaced with several different synonyms to accurately convey the intended meaning. Some possible synonyms include "departing," "starting," and "embarking." Alternatively, one could use the term "setting out," "heading out," or "taking off." These synonyms all imply the beginning of a journey or activity, and can be used in a variety of contexts. For example, one might say, "We're off on our road trip," or "She's setting out on a new adventure." Choosing the right synonym for "off on" can help to add precision and clarity to one's writing or conversation.