If you have the island unlocked you can go there in the night and catch the expensive bugs on the banana or coconut tree's... Otherwise I hear selling Turnips and Perfect Fruit is a good way to make money
A couple of questions - have you unlocked the island? Do you mind TT'ng?
The fastest way to make a million bells is to gather shells, fruits and fish until you have 1000 bells and visit the island after 7:00pm.
Go after the big sellers like golden stags, Hercules beetles and sharks. you can make approx 300,000 + bells per load up. Then you have the capital for turnips - the stalk market is really good for paying off those house upgrades.
Catch bugs on the island at night
Sell unwanted items on the retail forum
Check the train station forum to see if anybody's retail has your native perfect fruit on premium
Buy turnips on sunday's & check the forums throughout to week to see if anybody's retail is buying them for a high price
Go to The TBT Marketplace...everyone's buying them there. You can make 900-1 Mil per 100 TBT. I was buying but I'm not atm...but lots of people are looking for TBT.
You can sell your TBT bells for ingame bells! You just come to an agreement with the other person about how you're going to do it.
I paid someone 150 TBT in exchange for a bunch of sloppy items; I donated the TBT to their forum account, then they opened their gates and I visited their town and picked up the items. No idea why people want TBT bells so much but hey, if you've got 'em someone will want 'em.
Like Froda said you can sell your tbt bells. Last I saw at the TBT Marketplace 100 tbt bells equals to 1mil in game bells so you might be able to get able out 2mil for what you have. There is also the island bug method and turnip method. Selling your tbt bells is the quickest way out of the options and if you do well in the stalk market you can use the money you make from selling your tbt bells towards that. I wish you luck regardless of what you decide to do.
Edit: GaMERCaT has a good idea with breeding and selling gold rose. It comes down to you having the time and energy for it. You can get more breeding hybrids in general too. You can grab stuff off the island to sell (flowers, fruit and other stuff in the mini games), but I never found that really worth it.
Edit: Also you can use your tbt bells to buy rare items that people are selling and then sell them for in game bells. They average 100 tbt bells, but I've seen some hard to get DLC for as low as 50-75 tbt bells.