My fun is building real-life usable software and seeing people getting value out of them, so I guess YMMV. I'm very lucky that I really like doing this, as it makes work as fun as my hobby, and my personal portfolio has a bunch of finished (if generally small) products in it.
I enjoy this too, but predicting who is going to like what exactly is still a mystery to me. Programs I built that I think are the most beautiful and useful thing around get ignored or just tolerated and some silly bash script or Word macro I wrote in five minutes as a one-off for someone has been passed around multiple times and is still in use 20 years later.
Where I get my fun is that I live to increase my understanding of something. This code should be doing "this", but it's doing "that" instead. Is there something I'm missing in my basic understanding of this particular topic? When I follow it through and get that "Ahh, I get it!" moment, it's like a drug for me. All is suddenly right with the world again. It's why I gravitate to lower level languages and languages that give me more control.
The rest is building scaffolding and applying standard recipes and carefully laying things out as I go. I still enjoy it, but I don't get the happiness that I get from getting one of those "aha!" moments. Or being told my stupid Word macro saved someone hours of work in some random meeting somewhere, I guess.
