What was in the barrels that Charlie stopped to drop off?

The barrels had fuel in them. Camoflaging is the usual practice to prevent them from being seen by other airplanes and then stolen.

Bush pilots, meaning those pilots who fly in remote areas like the stark areas of Alaska and vast parts of, particularly northern, Canada, as well as the outbacks of Australia, amongst many other places, will store fuel, well-protected food, and other supplies along their routes. Oftimes it is just for them, so they will camouflage it, but there is a "gentlemen's bush flying agreement" that should you "borrow" some you replace it.
Because anything can happen flying those far-flung places, it always good to have extra for emergencies and / or just to get back.
Also, consider that when flying in the bush, especially revenue flights (making money vs recreational) weight is king. The more you can use for freight or passenger hauling the less you can carry on the aircraft regarding extra fuel, etc. The more freight/passengers you carry, the more money you make. Another consideration is that many of the places a revenue bush flyer will go to do not have fuel on hand, so you have to store your own along the route you usually take.
Sometimes those barrels will hold fuel, but they will also hold other things too. Mostly fuel though.

"... it would have been more effective if it would have resolved back to his origin and shown a few simple scenes to show how Charlie had evolved and carried it back to his world and recognized that indeed, unlike his earlier statement to the drunk Inuit, he had come to realize that we are all brothers... "

That could be so, if it were done just right. But I think one could just as well argue that Charlie's transformation was made even more powerful by the fact that the filmaker left something to our imagination. We did not get a predictable ending scene with Charlie saying, "Yeah, I learned my lesson all right" or something to that effect. I could go either way, but I kind of like it the way they left it.

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