Mario & Luigi: Superstar Saga
Game Boy Advance
As Luigi, turn Mario mini with your Luigi's Hammer, then hi-jump on Mario, but before Luigi gets on Mario make Mario jump, hitting Luigi. You will gain a coin. Repeat this task and you will be rich before you know it!
Go to Hoho Mt. Then let Mario drink at the fountain. Go down to the lower area. There will be a fan. Squirt water on it to get a coin/mushroom. Repeat for more coins.
Thanks to Revolution readers A2K and Tony!