Plastic bag

Not just good for carrying things in Plastic bags are also good for keeping things dry when it rains (you'll thank the Cunnan editors for this advice when it happens :).

You can usually get double strength garbage bags from large supermarkets (the heavy orange ones work well) and you will need at least two if you plan on properly sealing a backpack inside (and a third if you plan on crossing any rivers deeper than your waist).