Buy cargo for 50 Galleons from any Shipwright and sell it at a distant port. Runs between Ravenna and Sameria or Shell Island are highly lucrative for both XP and money.
Always your priority; they provide the largest chunks of XP and unlock essential map areas.
Completing 100% of an island's tasks (found in the map menu) rewards a significant XP bonus. Money Making (Galleons & Gems)
Digging up treasure provides massive XP. Completing a single high-rarity chart can sometimes grant enough XP for 1–2 full levels.