Stick to the main quests (with ! above NPCs heads) its how I got gil n exp the fastest, leves are OK but there is so much travelling in between and the rewards arent really that much IMO but its definitely better than just grinding

Also suggest becoming a Gatherer (Botany/Fisher/Miner) you can extract raw materials through their little mini games, but would take the focus off leveling your battle classes though, its just another way to make gil here n there.