Some synonyms for the phrase "for old times' sake" include "nostalgia," "reminiscence," "throwback," and "memories." "Nostalgia" refers to a sentimental longing or fondness for the past, while "reminiscence" refers to the act of recalling past experiences or events. "Throwback" is often used as a noun to refer to something that is reminiscent of an earlier time, while "memories" simply refers to experiences or events from the past. All of these synonyms imply a desire to revisit or remember events from the past, often with a sense of nostalgia or sentimentality.