Balloons are HIGHLY dangerous when inflated. A burst balloon can easilly cause an embolism and be fatal in seconds. NEVER put any inflated object in your body. If you want the feelings of marbles inside you, but in a space, then you would be better off using something like a cigar tube. They come in metal or plastic. You get the same sensations and no danger.
PLEASE NEVER PUT A BALLOON IN YOUR VAGINA OR ANUS.