CherryTree said:
Go to another town and pick some fruit that isn't your town's fruit. Go back to your town, plant a bunch of trees, then sell the fruit. Each fruit sells for 500 bells. You can also go to another town and bring your town fruit and sell it to the nook in their town. Again, 500 bells per fruit.
If you cannot visit another town simply send your villagers letters with fruit attached, also sometimes your mom will send you fruit. And you can find Coconuts on the beach.
Also try collecting bugs during the summer (at night, i find playing at 1am gets Scarab bettles which are worth 16k) and fishing during Winter (fish during a snowy night to get a coelcanth!) will get you money.
Also finding fossils will get you some bells >Note< I often donate fossils so that i can build up the muesuem first then sell them when the curater wont take them.
The only other ways are turnips, Red Turnips and planting money trees (bury 30k with a golden shovel, if it works you'll get a tree with 90k on it).