'swiss ball' exercise has been commonplace for some years now, often in conjunction with pilates movements, as the unstable surface of the ball helps engage your postural muscles more intensely. There is a risk attached to the instability however, so its best to get qualified instruction on their use, or, if you must go ahead, be very careful of your home environment where it is placed: lots of open space, no sharp corners or edges within falling distance. Matting or thick carpet on floor etc. In all seriousness, people have died falling c0ck over cranium off of these things, so be careful.
As for fencing, I did a few lessons at university, its great exercise, and a fascinating skill, but you dont need to worry about the weight of the sword, they are not very heavy, and a well balanced foil will be manageable for the most waife-like of ladies.