Here is an exercise that develops core strength and balance. It is basically a creative version of a baby bouncer chair. The bassinet is on a rocker so that the baby’s movements are translated into a rocking motion of the rocker. Because this makes it more difficult to balance, but also more fun for the baby, it is an amazing exercise. The baby realizes (in one way or another), that even tiny movements are magnified because of the unsteadiness of the rocker. When learning how to control a movement, this magnification is really helpful, allowing baby to zoom in and get more feedback about how to control the individual muscles. It is like using a very precise ruler rather than taking a rough estimation.
After a few minutes of experimenting with the rocking motion, you can stabilize the rocker with the stopper and baby will suddenly have very sturdy and accurate balance. It is good to give your baby different sensations, like rocking and still, so that all the muscles can respond appropriately.