Sitting in an Unstable Equilibrium
Sitting on a chair makes us far too static, causing us to fix in our body which is what it is not designed to do. Sitting is a balancing act, just like riding a bike, if you cannot peddle it is really hard to balance. So why not sit on something that is unstable but maintains some equilibrium. Here is one - a wobble cushion. It is smooth on the other side if you don't like to knobbly side. Excellent substitute if you cannot sit on a gym ball.
Here is a link to buy one