Easy Money:
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!

Unlimited Mushrooms/Coins:
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!