Talk to your neighbors. Sometimes they ask you to do them a favor and in return, they sometimes give you bells and sometimes they give you items.
You can hit a rock (you would have to see which rock that is.. everyday the money rock is in the different spot) with your shovel.
See if you can spot gold stars on the ground. Dig them up. It might be a gyroids (fire hydrant looking) or a fossil. You can sell them to Tom. Fossil, you have to check them in first at the museum. The gyroids.. if you complete a set of the same family.. it will make a tune in your house if you display them in your house.
You can catch bugs and butterflies with your bug net. You can fish with your fish pole. All the equipments you need, Tom sells them.
Shake the non-bearing fruit trees. They give out 100 bells. Some trees give out items (it's in a form of a leave when it drops on the ground).
Balloons.. shoot them if you see them with your slingshot. It will drop and there's a present attached. You can sell that item at Tom's.
Good luck.. and have fun!!