What I did recently is make an investment (such as auctioning a full collection of furniture, (in my case, Princess Furniture), which I included in a bundle with the small crown (that is 1.2 Million in itself!) And some other things like a Princess Dress and reorder-able items with the similar theme to match, like the Lacy Parasol, Blue Vase, etc.... I first set my auction to 5 Million minimum bid, and people said I set it too high, so I re-made my auction with a 3 Million Bell bid and the first person to bid offered 10 Million. :O Which to me, was an insane amount of bells, but I guess it was in really high demand. They seemed really appreciative for it too, so that made it all the more worth it.
In the process of receiving feedback from my first unsuccessful auction, (where I set the min bid at 5 million) people said that if I wanted to make money, I should sell my TBT bells because the market is inflated right now. I didn't know the going rate, so I was hoping for something like 100,000 bells for 100 TBT Bells, and suddenly people wanted to buy all 600 for 5, 6, and then 7 Million Bells all together. Also insane, to me, but I got a message from someone that I should wait it out and expect more since the prices are so inflated. Thanks, but no thanks, 7 Million is more than enough!
In terms of earning in-game, I have every kind of normal fruit in my town. Every time my tress grow fruit, I harvest them and then stockpile the baskets of fruit until Retail buys them at premium. Then I had the problem of having wayyyy too much fruit! So I have stopped harvesting and just waiting to sell the rest. Sometimes I offer fruit as tips, or just gifts to people who have been kind to me on here (which has been pretty much everyone!)
I also support shops on here as well, and buy hybrids even if I don't necessarily need them to help others make money too. They usually tens to see what I have to offer as well, like furniture series, and then I earn bells too!
Bug and Shark hunting on the Island at night is ALWAYS a good choice too!