Make deliveries to various merchants each day so that they have stock to sell to customers. Earn more based on how many boxes you deliver, and affect the economy based on who you deliver to.
Work for the merchants, haggling with customers to earn a profit. Save up to purchase your own stalls to start growing your own businesses on the side.
During your time off (in the evening and on free days), you can choose how to spend your time. Whether that's catching fish, diving for clams and treasure, or figuring out the mystery of your past. Just make sure to pay back the Dock King .. or you may be in for a rough time.