Leg weights are the way to go Or if your going for a brisk walk, walk with dumbells in your hands. My main concern with a bag of sand in your bag is that running around with a back pack alone, sand or not couldn't be the most comfortable thing, and annoying. All bouncing around and stuff. *shudders*
sand's a sensible idea. make sure the pack doesn't slip around whilst running with it on your back and don't put anything with corners in the back pack like bricks or anything ~ you'll know why if you try it.
Good judgement comes from experience.
Experience comes from bad judgement.