When a character named Joan shows up you can by turnips and sell them in bulk for profit. Just have to browse Animal Crossing forums like this one looking for someone whose Re-Tail is selling them for around 400-700. You can get badges from Phineas for selling enough turnips, and it is a 'quick' way to make bells. You have to be careful not to time travel when holding turnips, as they rot.
In regards to decorating, when Lyle shows up in Nook's Homes you can get more information, but basically matching sets, matching colors, rarer items (e.g., villager pics), and certain colors on certain parts of the room increase your score so that you get rewards from the Happy Home Academy.
To spawn more beetles definitely more banana or coconut trees. If you want a higher chance at a rainbow stag showing up, though, then the other trees (the ones that grow on grass).