Meet the PKB Kettlebells: soft, adjustable weights that let you work on your body and get fit. It is compact and does not damage your floor. It has comfortable handles, so you can perform various moves. You can fill these with sand, water, dirt, or rocks (from 0 to 45 lbs).
The PKB is made of ballistic nylon. Since it is soft, it does not put unnecessary stress on your arms. You can get yours for under $32.