After you complete the “Take Back the Sea” side quest (which is as simple as just killing a bunch of bokoblins on a nearby beach) and restore Lurelin Village, you’ll find a house in the northeast of the town where you can gamble your rupees for a quick buck.
Where to find the gambling house
Travel to the Yah Rin shrine and head down into Lurelin Village. Keep heading roughly northeast as you follow the paths through town. You’re looking for a house with the door open — you can see it when you look northeast from the Lurelin Village inn.
How to gamble
Bear in mind that this is by no means a get rich quick scheme. When you walk into the house, talk to Cloyne. He’ll explain the rules of the game to you — you pay 10, 50 or 100 rupees for a one-in-three chance to win money. Buying in at 10 or 50 rupees gives you the chance to double your money. Paying 100 rupees, on the other hand, will triple it.
After you buy in, all you have to do is go pick a treasure chest. There’s no way to game the system here — you’re just at the mercy of chance. Two of the chests contain a single rupee, and the third has either double or triple what you paid in.