I have yet to really do one strictly "For fun"
The closest to "for fun" I have ever come was back in high school, a couple of days after I first got my 4-qt "great white" open top bag, Admittedly the only reason was to try out the bag.
That one just was not necessary, but I was eager to try out that bag!!
The other time that can be said to be "for fun" was when my boyfriend (now husband and father of my children) and I first started really seriously discussing marriage. I had come clean to him about my fetish a couple of days earlier, and was expecting to get a ring any day, or now that he knew, maybe a break up, I was not too sure. I gave him an enema just to test his reaction. Only enema I have ever given to anyone else, though he HAS self-administered a few times along the way. Again not really for fun because if he is going to marry me he is going to have to tolerate certain things.
Right after that one was over, I thought I had blown it, but he actually gave me the ring later that day!
So, of all the enemas I have ever done those were the least purposeful of the two and therefor could have been said to be "For fun" though neither seemed like it at the time