In my experience, the easiest way to get a bunch of bells is to just collect a bunch of fruit from Tortimer Island and sell it back at Re-Tail.
The best way to do this is with the diving tours as those have a lot of fruit in the beach area. Exotic fruit is worth a lot so just keep replaying those until you fill up the box. It takes a while but i completed several projects in just a few days by doing this. You can also just plant all the fruit you get from the island and eventually you'll have more to sell in addition to the island fruits.
Another thing you can do is hit every rock in town with a shovel. If one starts dropping bells, keep hitting it to get the rest. You might also get ore from rocks and those are worth a lot of bells. The money rock and the ore rock change locations every day so keep trying until you find them.
And a few more things:
Do as many town initiatives as you can, MEOW Coupons can be exchanged for bells at the post office and are worth 3,000 each.
It's also worth talking to Wisp every day if you have a compatible Amiibo, as he will give you 5 coupons after granting a wish.
Once you unlock the Dream Suite, it's worth updating your dream town every day as Luna will always give you 5,000 bells for doing so.
Once you can start working at the Roost, just look up all the coffee preferences online to get a perfect score. This is important because then you'll be paid with Superb coffee beans which can then be put up for sale at the Re-Tail flea market for exactly 7,999 bells.
Bring every fossil you find to Blathers. If it's one you already donated, sell it.
And finally, just simply catching a lot of fish and bugs and selling them will net you a nice amount of bells.