Go to the casino level. It helps to get rid of any black pants wearing monkeys in the first part of the level. Break open all of the slot machines in a counter-clockwise direction. After awhile you will meet a rat-looking enemy that takes your money when you run into it. Let him hit you and allow him to take your gold. Then, quickly hit him and all of your gold will be dropped. Grab all of it quickly; after you collect 5 gold pieces they turn to silver stars. After 5 silver stars, they turn into gold stars which are worth 10 gold pieces. Continue hitting the slots and you will find one more of these rats. Also, after you defeat Spector (Mech-Monkey Sub-Boss) and unlock the extra monkeys, go to the Simian Citadel level. Toward the end of this level, you will find a wizard-monkey named Tommy. If you do not catch him, he constantly creates enemies. One of these enemies is the rat thief. Let him steal your coins, then steal them back. You can use the Super Hoop to catch him. Keep repeating this until your coins are at 999. Note: even if you catch Tommy and save the game, you can still return in time trial mode to perform this trick.