Oh, well I think we use both words the same way then. :D
I'd have to argue that anything you choose to pursue out of interest would have to be in some way, to you, fun. Fun in a specific (and kind of sad) way, admittedly, but I'd say fun nonetheless.
I dunno, I tend to think of "fun" as something that makes you happy but that also has an element of excitement to it.
I find some coding exciting :-$