HK, for starters, go to the laptop on your desk, and use Coin Miner. It uses up a full time segment, though. Next, go to the beach and gather shells. Also look in the Woods and gather cranberries and flowers. They show up at random times of the day, and vary in type. After you gather some up, sell them to Cindy or Moon at the Mall.
You can also work for Cindy in her shop.
Finally, if you have access to the Realm Of Heat, go mining for gems and metals to cut/refine and sell.