Meet Aqua Bags: water-filled bags that add instability to your workouts, pushing your body harder. The fuller it is, the more stable and heavier it gets. When it is light, it lets you perform swings, curls, step ups, rows, and cardio moves in a more challenging fashion. You also get adjustable handles to try various workouts.
These bags are available in 45 lbs and 70 lbs versions.