Around that age for my son, it was an imaginary mouse. He'd hold out his empty palm, and say, "You want to pet my mouse? He's very soft." It was the center of his imagination for quite some time.
For me, I wish there were a single focus. Instead, I think writing software and being with my son wrestle for my attention. I wonder if, when he's old enough to write software of his own, of he'll even want to, and if he'll still be so enamored of spending time with daddy.