For fast cash, I just use the Zen Garden and buy flowers, sell them, repeat. And I set my date on my device to the next day to make them grow faster. There aren't that many things to buy in the shop so this method works. But for things that need WAY more cash like growing the Tree of Wisdom, setting up a perfect Survival Endless strategy is probably the best way. Once you get to very high flags, you get TONS of money, and can set up Gold Magnet to help collect coins. (Plus late game you're gonna wanna grind survival endless anyways. There aren't that many things to do late game besides Endless Mini Games and creating intricate Zen Gardens.)