Second the duffel bag suggestion. We have a nylon rip-stop one that folds up into the size of a shirt basically. We take it with us everywhere (usually for dirty clothes) but when we go back to the USA it's one of our cargo carriers. You can also just get a few el cheapo bags at Walmart (they have a big duffel bag with wheels) and fill it with stuff.
If you are bringing more fragile items you may consider some hard-shell suitcase. But for soft things like diapers and clothes, a duffel bag is a great way to go.