Hunting and gathering
Unlike in Far Cry 2 (and most games, for that matter), hunting in Far Cry 3 actually serves a useful purpose. There's wildlife everywhere, from boars to cheetahs and komodo dragons, and gathering their skins gives you the raw materials to make bigger backpacks and more useful equipment.
On top of that, harvesting plants around the island lets you craft health-giving tonics and, presumably, more useful concoctions that we didn't get to try out. Maybe the drug-addled but likable Dr. Earnhardt, who serves as both safehouse operator and narcotics manufacturer for the island, can help out in that department.