Right now, my idea of fun is 8 hours of uninterrupted sleep.
In the years BK (Before the Kid), my main hobby was practicing jujutsu. It consumed much of my already scarce free time: I trained 3 times a week, and had day-long practice sessions once or twice a month. I also played pencil-and-paper RPGs once a week with friends.
Those were the main things.
(Well, I guess I could add going at the movies, dining out, or grabbing a beer with friends, but I'm not sure it could be classified ad "doing something"...)