This is a popular followup baby game to Pony Rides. It will really develop your baby's balance and movement coordination. Your baby should be able to sit on the floor unaided with good balance before doing this activity.
Sit on a chair or couch with your baby. Seat him on your knee or thigh facing towards you. Keep your hands next to his torso (under the arms), ready to catch him if he loses balance. Move you leg slowly back and forth then gently up and down. You can also move your leg in circular motions. After a few minutes of play, turn him around and repeat the game.
As your baby becomes more adept, you can increase the speed and amount of movement. Another variation that your baby may enjoy is to seat him on your stomach as you lie down on the bed. Remember to keep your hands close and ready for off balance blunders!