Those are some good suggestions. Also I've read elsewhere you can use a plastic milk container, put in some water. I guess a screwtop would be less likely to spill water. & one can put in some rice once the water filled one is to light. Though it will be a bit awkward since the weight will be mainly on one end. Maybe more awkward if it's only partially full with the water or rice moving around as the bottle is moved around.
Also maybe a backpack with some books, or other things, can be used. Especially if it has a strap like mine does. Besides the 2 normal straps to go over the shoulders it also has a strap at the top of the back where you can hold the backpack by one hand.
Not sure how good they are but maybe some plastic bags also, with a book or 2 in them. Though they may not last long and may be awkward to use. There are other alternatives like these and the ones mentioned previously by other folks.