Here is a basic rundown of some of the money making activities:
Fishing (The entire ocean)
Wreck Excavating (Underwater Wrecks)
Picking Fruit (Apple, Peach Fields)
Mining Diamonds, Iron, Copper and Sand (Quarries)
Oil Running (Oil Rig)
Vehicle Crafting (Vehicle Factory, Black Market Production)
Drug Running (Weed, Heroin, Cocaine, Meth)
Bootlegging Alcohol (Spirit Distillery)
Weapon Crafting (Black Market Production, Firearms Factory)
Bounty Hunting (Bounty Hunter Shop)
Animal Hunting (Hunting Grounds)
Robberies (Gas Station, Federal Reserve, Bank of Altis, other Players!)
Police Officer (See Police Application on Forums)
Medic (Medic role pays for reviving other players)
Delivery Missions (DP1->25)