I used to catch bugs, and I still do, but I swear, selling turnips is the way to go.
Thanks to ACTurnips on reddit, there's usually a town that has Reece buying for 500+ (the people there are really nice too!)
I made a good 8mil yesterday thanks to that
I like going to Tortimer island at night, making sure nobody can join me by asking to go on a tour (but not actually going), then demolishing it and catching beetles until I fill my whole box with them
It's a great way to make lots of money, if you have some time on your hands you can make a million bells in one night.
In the Summer, I also enjoy during the daytime catching bugs and fish in my town- it's not as efficient as island beetle hunting, but catching lots of butterflies and different fish can pay off eventually.
Going to the island to beetle hunt is a great way to make bells but ever since I bought a second copy of the game to use as a cycle town I hardly ever go to the island.