Early game there are a lot of different paths to making money. Nothing is going to leave you rolling in dough, but everything will make you a little. Fishing is one of the most commonly recommended, but it can be tricky for new players to get the hang of. Growing crops to sell, raising chickens for eggs (and then mayo), going to the mines and selling the gems, and finishing quests are all great ways to make money early game. Even picking up forage can make some money, though I tend to use forage-ables for food and/or gifts more often than selling.
Mid game, transitioning to processed goods will open up much better sell values. Mayo, cheese, pickles, wine, cloth, smoked fish, etc., all have a lot higher sell value than their raw forms. You'll also start getting access to the mossy logs (I think they're called? Mushroom logs?), batteries, and ore from skull cavern, which all can be sold at very nice prices.
Late game, everybody has their favorites, but aged wine is a favorite standby for a lot of people. It's relatively low time investment once it's set up, while giving a very nice payout.