Credit: -Kenthos-
Preface
As it's quite often that a new player asks for tips on making money, we decided to compile it here. We will list all the possible activities and how to do them efficiently for making money, as well as the upside and downside of each activity. You can then choose which activity you like the most.
Farming
The most obvious choice here. Everyone will tell you to grow pumpkins if you choose farming. If you're early in the game, tomatoes or carrots are your best bet. Onion or Canola if you've unlocked Forest of Valor, and Okra if you can access Glade of Trust.
The best part about farming is that you can level up your villagers very very quickly if they hang out with you the entire process of farming (i.e. planting - watering - harvesting). Also, you can do other activities while you wait for your crops to grow.
Foraging
Your main source of money will be the fruits and the herbs if you choose this activity. You can move all of your fruit trees and bushes to one place to make foraging a bit easier. The good thing about foraging is you don't need any energy to do it. Also, you can level up your villagers if they hang out with you while doing this activity, albeit slowly. The bad thing is that the foragable resources take a while to respawn, thus you can't just forage constantly.
Fishing
A relatively fun activity. The best part about fishing is that you can do it endlessly without needing to go back to your house. Just eat one or two of the cheapest fishes you've already caught and you're able to fish again. This is assuming you rarely fail to catch a fish. This activity also levels up your villagers pretty quickly,
Cooking
There are some recipes that's literally just you buying ingredients, mixing them up, and BAM profit. The most famous example is Soufflé. You buy cheese, milk, eggs, and butter (all available in one place), cook it, and gain about 400 coins profit. The best thing about cooking is it's very fast and took little effort. However, it's a bit repetitive and might not be fun for you. Also, you need to keep in mind your coal supply, and that cooking won't level up your villagers.
Mining
Gems give you a lot of money and the shiny ones even more so. This activity also levels up your villagers the fastest. You can go mine every mining spot in your village in a loop (there are 6 of them in each biome, plus 12 in the Vitalys Mine), each of them respawns every 5 minutes. However, this activity depletes your energy pretty quickly and unlike fishing, you can't eat your gems and stones. To solve this, you need either prepare in advance some meals for you to eat on the go, or you need to constantly go back to your house. Also, gems are random so you're not gonna get them from every mining spot.
Digging
Well, nobody actually recommends this but hey, it is a way. Digging occasionally gives you a small amount of free coins (under 10 coins) and you can sell anything you dig up too. However, they don't sell for much and you probably want to save them up for quests anyway. Digging also depletes your energy quickly, just like mining.