Honestly, parenting is the most challenging, difficult, wonderful, awful, exhilarating, scary, enjoyable adventure I have ever undertaken...and I enjoy it immensely, even during the very times I am hating it at the moment!
Other than that, I would say learning to play an instrument...and then getting past the point where it seems like work, and it becomes pure joy, and you get into a "zone" when are engaging in playing the instrument...it becomes like an extension of your body.
Not that I would ever be good enough to carry his guitar picks (!), but if you have ever seen Eric Clapton get into the zone while playing his guitar, THAT is what I mean.
When I see him play, I want to smash my guitar against the floor because I know I will never be good like that, but I get into my own little amateur zone, and I feel fine!