Ronfaure region, San d'Oria
The forests on the Ronfaure region are perhaps the worst of the three starting areas to level up in terms of crystals and other items you can sell off, but leveling is just as fast as any of the others.
Any monster that won't slaughter you outright drops cheapola Water or Earth Crystals, so attacking everything you see around the gates is probably the best course of action. In addition to Earth Crystals, Wild Rabbits have hides you can trade to a computer-controlled character in South San d'Oria for money. Tunnel Worms may leave you Valuable Zinc Ore.
Around level 5, you'll be able to diversify your prey. Funguars, creepy frog-like mushrooms, will net you valuable mushrooms as well as Dark Crystals, which are in high demand by San d'Oria's Leathercrafters. Orcish-type monsters will attack on sight, but should be safe as long as they "Check" Even Match or lower, and will drop valuable Fire Crystals.
Having attained level 8, check your map and head to Lufet's Lake in West Ronfaure. The River Crabs here will not only give you solid experience, they also drop Lufet's Lake Salt. If you collect three of them, you can complete a quest in Port San d'Oria.
Other viable targets are Wild Sheep, which drop valuable Sheep Hides, Goblins, all Orcish and Beetles. These higher-level monsters can carry you to Level 14 or so, should you have the patience to stick with lower experience.