Breed hybrid flowers or wear the dodo shirt when traveling to mystery island which is likely to get you hybrid flowers. Plant them (make sure it blue, purple or black flowers) and watch pea**** butterfly, sunset butterfly, rajah butterfly, arias butterfly fly by. Most of the time it just pea**** butterfly but they sell for 3750 when Flick come to town. A full inventory (40 butterflies) is around 150K. If you catch say 80 butterflies a day (they respawn every 30 minutes I believe), that 300K right there. Far less exhausting then catching golden stag on Tortimer island.
What I do is my hybrid flowers garden is right next to the beach. So I wait for balloons every 10 minutes walking along the shoreline, at the same time digging for clams, fishing, and watching my flower patch for rare butterflies.
That my secret to making money in this game. Otherwise I would be broke. I lost 500K on turnips this week. Getting hybrid flowers island (blue windflowers) was the best thing to ever happen to me money wise in this game as I discover a secret that I wouldn't have otherwise. Prior to that I was making 50K a day. Now I could make 300K a day if I work hard enough. I rarely do though.