Unlockable Challenge Modes:

Unlock all Gaddgets and Mini-Games to turn the title screen gold.

To select your own background, buy a Poster Gaddget with the desired image in Challenge Land. Go to Play Land, choose "Gaddgets", then select that poster. Return to the title screen after playing with it.

Play all the Koopa Kid mini-games. Note: This requires to game cartridges to play.

If you have unlocked the Watch 'Em lottery game, go to the game room and go play it. Put 99 coins in, and get the first one correct (1 of 10 chance), the second one correct (2 of 10 chance), and the third one correct (3 of 10 chance), then press B and go back and do it more times. Note: Every time you do this, you will get slightly less than 800 coins.

Start the game with Peach. Go to the Horror area and then to Mr. I's house. Keep talking and you will complete the quest and get a new gadget. Note: Make sure when he asks if you are Princess Peach to say yes or you will not complete the quest and get the gadget.

Unlock Pair 'Em, then go into the game room and play it. Put 99 coins on everything (press A then R to make the bet go to the maximum). If you are good, you should be able to win any of them easily. However, if you win all five things, your multiplier will double and you will win 4,752 coins each time.

To see the credits, earn 2,000 coins in Challenge Land. Then, go to Challenge Land and select the last option, "Trade coins for gadgets". Buy the credits. Go to Play Land, go to "Gadgets", and press L. IT is the option at the bottom-right corner.

After unlocking the "Soil Sonar" Gaddget, play with it. Sometimes, when you go deep enough into the ground, two odd statues will be there. On closer inspection, these statues are Gyroids, which are found in Animal Crossing.