Buy some physical gold, wait for it to have a bull run prior to a US$ devaluation, then immediately after the devaluation sell gold for US$.
My tip is the world's central banks will buy US dollars and sell their own currencies to haul the greenback up again. It will be very hard to keep the greenback down (even if it should by rights be down) because the other countries need a strong dollar to allow the US to keep consuming their goods.
