The phrase "creep up on" can be replaced by several synonyms depending on the context. For example, "sneak up on" is a common phrase used when someone approaches quietly and unexpectedly. "Surprise" or "startle" can also be used when someone is caught off guard by something or someone approaching. In a more positive context, "gradually approach" or "slowly advance" could be substituted for "creep up on" to describe something approaching in a gentle or peaceful way. Ultimately, the choice of synonyms depends on the intended tone of the sentence and the specific situation being described.