Digging up fossils: Once you find a weird looking star thing on the ground (which means something is underneath it, ready to be dug up), go ahead and use your shovel on it. It will either be a pitfall seed, a wooden statue thing, or a fossil. When you find fossils, go to the museum and show it to the owl. He will say what kind of fossil it is, and by now, you can sell it to Nook. Do not sell it before you show it to the Owl, it will be far less money. You can also choose to donate fossils to the Owl too. Fossils are a very good source of money.
Shaking trees: Go around town shaking trees. Sometimes furniture will fall, sometimes money, sometimes bees (run for a building if that happens!)
Go visit another town: If you have a friend's friend code, meet up with them in their town. Shake their trees for their fruit and pick it up. Go back to your town and bury it to make a tree of that fruit; eventually sell the new fruit. Selling different fruit than the fruit in your town (for example, if your town has peaches and your friend's town has apples) will up the price. Your town's fruit is 100, fruit of a different kind is 500.
Find a coconut: Go by the beach and check for coconuts. If you find one, bury it to make a coconut tree. You will have to water trees every day so they grow. These trees take a while to grow, though. Coconuts are worth 500.
Hit rocks with your shovel: This is a hidden secret to the game. Every day, there is one rock in your town that holds money. Go around town and hit each rock. One will pop out some money. If you get money, keep hitting it until it stops giving. If you wait too long, though, it will stop. Your character will bounce back after each it, so to ensure you get the most hits on a rock, dig holes around you so you don't bounce back, therefore wasting time and not getting as much money.
Ex: http://oyster.ignimgs.com/mediawiki/apis.ign.com/animal-crossing-wild-world/thumb/c/c1/Acww_04_secrets_02.jpg/225px-Acww_04_secrets_02.jpg