I regularly carry a pack loaded with 'ballast' rather than my actual gear, clothes and food. I use several bulky items that completely fill out the pack to right about 20 lbs. That's about an overnight load for me, which seems right for my training regime, even though I often am out for a week at a time. I don't want to risk an injury from a heavier load because I train by walking faster than I normally hike, but fewer miles.

I would hesitate to use water, because water is much denser than a normal backpack load, which will give it a very different center of gravity than a normal load. Also, a leaky water container could be very unpleasant. I can't say if cat litter would replicate the weight and bulk of a normal load very well, because I don't buy it or own a cat, so I'm not really familiar with its characteristics.