This is something either old schoolteachers or prententious slappers use. To be fair, not everyone who uses this phrase is either or, but sadly most of the time this is said with a certain intonation to show false delight in activity, etc that the other speaker may have just named...basically a more subtle way of saying "oh the things you get up to are so quaint and CUTE!!!"
Person 1: "How was your weekend, sweetie?"
Person 2: "Well mainly I just stayed home, though I went paintballing with my friend one day."
Person 1: "Oh how fun!!!!!"