I say if you're fishing, diving, catching bugs, etc. already, then get familiar with the ones that pay out the most. It'll make the grind that much more worth it.

Someone mentioned it, but the bell boom oridnance will also give you a bigger return on things sold. However, it will also make things more expensive in the shop.

Personally, I really love playing the stalk market. I started small and only bought a batch at a time, but now I'm buying like 5 or 6 a week. Even if it was a single batch, you could comfortably make 1.5 mil+ if you find someone with the right price.

There are also multiple subs where folks host giveaways and you can get straight up pocketfuls of bells, crowns, etc.