Starting around 3 or 4 months old, I used to sit J in my lap in front of a full length mirror. He loved looking at the other adorable baby in the mirror. We sangs songs in front of the mirror, practiced hand motions, and we played with simply rotating toys. He liked to watch both the toy itself moving, but also the toy in the mirror.
Around the same time, I learned to lean a mirror on against a wall and lay J down next to it. When left to his own devices, he just chattered away to his new friend in the mirror. This kept him content long enough for me to mop the floor/cook dinner/iron.