There is no idiom in English that specifically means "fun for fun's sake," but there are a few idioms that might convey a similar idea. One such idiom is "all fun and games," which means that something is enjoyable and lighthearted, but not necessarily serious or practical. Another similar idiom is "fun and games," which can refer to activities or situations that are enjoyable and entertaining, but not necessarily productive or important. Additionally, the phrase "for the fun of it" can be used to describe something that is done purely for enjoyment, without any other purpose or motivation.