- Build all of the level 1 tents ASAP, they don't have to be displayed to increase the max level of that style's animals, everywhere in the world. Upgrade to level 2 when you have money, then level 3 when you have a lot of time (when you sleep, for instance).
I foolishly only did Cool and Cute level 1 first, then went ahead on Cool level 2 & 3, and had several Sporty and Natural animals stuck at max level 7 for 12 hours.
- Don't ever vend (sell to the bank) any fruit or the uncommon (100 bell) fish or bugs. Animals will request those at higher levels, and pay 10x what the bank does. If you have too many, put them in the Market Box at a reasonable price, under 10x price so people can make a profit!
I'm selling fruit & common shells for 40 bells each, uncommons for 400 each, and doing good business now. I vend most common bugs & fish down to 6 each, but sometimes put them in my Market Box.
- You can layout your nice furniture, and use Auto-Arrange when you finish an animal's invite quest, then Clean Up, and it'll restore your furniture layout. Do not sell furniture, multiple animals will request the same items.
- Hit the cat icon in your campsite to change who's stuck inside with you. I keep those who give good resources (Tex, Apollo) and leave out the ones I need to level faster.
- Check the Able Sisters market every 6 hours, it changes from Mabel selling shirts, to a skunk selling shoes, to Sabel selling pants and hats (is Labelle in here? I've not seen her). There's no dialogue story like in Wild World, where over the course of weeks or months you learn about their background.
- Check Timmy Nook's store for furniture you might need in a quest, far cheaper than crafting it. There's no furniture inventory limit AFAICT, shop till you drop!
- First thing in the morning, request help with the quarry from everyone on your friend list, and anytime you see a shovel icon by their name click in and Lend a Hand; you get 10 or 100 bells when they use it. Once you get 5 friends to help you, wait for a resource you want to be focused, it changes every hour. Don't waste leaves on the quarry, it's just a little cash and a stack of up to 10 resources.
- Shake all the non-fruit trees, one a day will drop a bag of 100-1000 bells.
- I always log out in my camper, so the area outside will be reset when I come back.
More detailed info at these longer, often too long sources: