Dig for fossils
Every day your island will spawn four new fossils - not three to five like you may have heard from elsewhere, those people clearly just missed one on one day, and then found it, in addition to the usual four the next.
Get all of the fossils, have Blathers assess them, and then donate whatever is new, and then sell or collect whatever is left. This is an easy way to make money and build up your collection.
Yeah, you can eat a fruit to have the strength to smash rocks or dig up trees, but you should leave the fruit alone and just hit a rock with two holes dug behind you. You can collect materials and 16.4k bells each day.
If you smash rocks, they will respawn at a rate of one per day at a random location on your island.
Collect everything. Fruit to sell? Shells from the beach? Weeds? Branches? Everything, just build up that collection of materials and new items to sell.
Going over the whole town can always bring extra rewards like balloons, can help with finding fossils, and of course you'll find a message in a bottle on the beach daily.
Check the shops
The Able Sister's and Nook's Cranny shops will have a new inventory every single day, and if you want to customise yourself, your home, and your island, you should visit both.
Don't just look at what they have on display, check the cabinet at Nook's Cranny for wallpaper and flooring, and the changing room at Able Sister's for a bigger inventory. Oh, make sure to speak to Sable, too…
Bury 99k bells
You know those golden spots you see in town, that you can dig up for 1k bells? Don't just bury the 1k in the hole for a 3k tree. Bury a 99k bag for a 297k tree.
Yes, it's a big investment, but the money comes back fast, and is well worth it. If you don't have it, just throw in as much as you can, and try to increase the amount each day until you hit 99k. Don't believe the "10k is the max" lies.
Look for special characters
You will often have special characters visit your town, either in front of Resident Services, or just walking around the island. They will always have something unique to sell, or a challenge to rise to.
CJ and Flick are great visitors, as they will give you exclusive fish or bug models in exchange for three of whatever model you want, and will also buy your fish and bugs, respectively, at a higher than usual price.
Nook Miles +
You shouldn't fixate on Nook Miles too much while playing, but it is a good idea to get your initial set of Nook Miles + challenges out of the way early each day, as you will get a nice top-up to the amount of Nook Miles you have.
Also, go into Resident Services to check what's on offer in exchange for Nook Miles and in the catalogue - you will also get a daily Nook Miles bonus for popping in.
Each day you can relocate a building, build a new bridge, a new incline, sell some more land, or whateever. You can do one of each of these tasks each day, and it's definitely worth doing, as long as you have the bells.
Getting these infrastructure upgrades in place makes it much easier to navigate your town (I don't need the vaulting pole at all anymore!) and makes it much more exciting to visit your town.
Water the important flowers
You should have a lot of flowers in your town, and watering them all should be more work than it's worth. Which is why you should have just a few important flower patches you keep up with watering.
Watering flowers helps them breed, and when placed next to other flowers of a certain colour or type, you can breed entirely new varieties, which can spawn exclusive or rare bugs, or perhaps just sell for a high price.
Gotta catch 'em all
Catch bugs and fish. It goes without saying, right? Bugs are great, because you can spot a bug worth a pretty penny from a mile away, while fish are more elusive. Still, if you see a big boy off the pier or at the mouth of the river, make sure to get the rod out.
Remember that logging in at different times of day will change which fish and bugs appear, and that if CJ or Flick visit, you should make catching and selling to them your goal for extra bells.
In fact, just log in regularly throughout the day to catch bugs and fish to make a profit. Why not, right?