Fun Fun Island, once mastered, is the fastest way to win rupees than I've ever seen. I managed to get from 2600 rupees to 9900 in just under 45 minutes. Wanna know the secret?
Here it is:
As you're falling through the last ring (This works even if you only go through ONE) REMEMBER the place where the 50 rupee panel is. If its directly under you after passing through the last ring DO NOT MOVE. There's a high possibility of landing on it when it comes back around, I've only had a handful of times where its past me but not by much. Keep in mind that literally any movement you make it gonna increase your decent, memorize the panel order and you'll be able to compensate bad positioning. For example, the 50 rupee mark was all the way on the opposite side of the wheel from where I was falling through the ring (I know, its hard to picture that lol) and I move Link up by 90 degrees and land right on it. Think of it as a clock and subtracting three hours; yes you fall VERY fast whenever you move. I've had times where I saw that I was gonna be off by a panel and just tilted to do a forced landing onto the 50 rupee mark.
Now I got a save file with 9900 rupees and I may do a video tutorial (Chances are, someone's most likely beat me to the punch lol). But yeah, if you master it, you'll be rolling in rupees in minutes.