
Selling beetles has always worked for me. ^^ Those things sell for a quite a lot of bells.
Selling popular villagers into slavery.
Growing foreign fruit, selling your BTB and turnips are all fairly decent.
I think bug catching is the best thing to do -- although that is possible only during the evening, so you can't do that much during the day to begin with.
Another thing is turnips -- you should invest well in those, and when Reese offers a high price (or you find someone in the forums that has high turnip sell value in their town) sell then.
Honestly I feel like TBT and such is somewhat cheating since it isn't an IGB earning methods, but I think it's pretty good too.
My way of getting bells is catching sharks and beetles, even thought it is VERY time consuming. I usually get around 400K but it takes me about 2-4 hours (I think) to fill the island box.
A method that you can do everyday but may not be as quick is finding money rocks and maybe selling fossil/gyroids and ores.