* Sell fruits
* Sell furniture on Flea Market Day
* Sell mushrooms (Only Applicable During November)
* Sell fossils (You don't need to give them to the museum.)
* Collect and sell sea shells (Keep the scallops for Pascal: he gives you a rare item for them)
* Let the townsfolk turn your Garbage (Pitfalls, cans, tires, boots, etc.) into Jack-in-the-Boxes on Halloween and sell them
* Deposit money in the bank and wait a couple days, you get interest on your bells
* Shake trees for 100 bells and an item each day (Watch out for bees!)
* Cross-breed flowers and sell them
* Collect special event furniture and sell it to Nook (Harvest, Mush, Snowman etc.)
* Take the items from the Dumpster and the Lost and Found and sell them
* Mail letters to townsfolk, they might send presents back

If I decided to add anything else, I'll add it in the comments. Others can add too in the comments.