Fishing and turnips. Right now the only good fish for northern hemisphere are mahi mahi, trevally, and golden trout (all rare as heck). Regardless of that fishing will still net very large bells if you set aside an hour for it. Then invest those bells into the turnip market on sunday, you're bound to find someone with insane prices, just be careful XD
you can start a fruit farm shake your trees everyday with a net, and they can drop 100 bells and 2 trees drop furniture every day. use a flimsy axe to harvest 3 wood from all trees wood sells for a reasonable amount. fish or go bug catching. sell unwanted items to timmy and tommy and if your really desprate i usually just... shake a tree for infinite sticks get a full inventory and do it again. however you dont have to do things fast animal crossing is a good game to take at your own pace. I hope this helps you it has helped me make some quick bells.