Halfway through Spring, you can attend the Egg Festival. Make sure that you've saved up a fair bit of money by that point, and find Pierre at his vending area. He'll sell you Strawberry Seeds, at a rate of 100G apiece, and you should invest whatever you can afford in that lucrative item. Then you should plant the seeds and harvest the berries they produce. Each berry is worth 120G when you sell it, and each plant can produce new berries every two days or so. It won't take long to recoup your investment.