Once kneeling up on the ball is easy, a new challenge to master is superman on the Swiss ball.
Start by just taking one knee off the ball and sticking out the leg, replace it, then slide the other knee of the ball and then stick out that leg.
When this feels a bit less precarious, try taking off the opposite arm too, as in the above video.
There is a very high likelihood of falling off the ball with this exercise. It suddenly shoots out sideways from under you. So do make sure you have a good space around you. Falling onto something will hurt much more than just spinning onto your back.
Do a challenging exercise like this at the beginning of a workout when you are fresh. If you attempt it at the end of a workout, you should be tired – or what were you doing during the workout? – so the chances of falling off and hurting yourself become much higher; either that or you will just find the exercise too difficult and get disheartened.