you may not think of it as much exercise, but i used to sit on a ball at my computer instead of a chair. it's a surprising amount of work and it forces you to have good posture and strengthen and tone your back and thigh muscles a lot more than you'd think.
i used to really enjoy doing crunches on my ball, too. they payoff was a lot higher, too, and i could get the same effect from doing far less of them on the ball than on the floor (or on the other side, i could do a lot more of them than i ever could on the floor and get results faster!). plus, no floor grinding into your back and hips. much better for your bones.
you can do absolutely anything you would normally do on th efloor in a modified pose on the ball. it forces you to align yourself properly, which the floor doesn't do.