Easy Coins
Instead of pounding the wooden posts found in some levels, circle around them three times to have five coins pop out for you. Fat Penguin Race
After getting all 150 stars, return to Cool, Cool Mountain and you can race a super fat penguin. Have Toad wear your cap!
While playing as either Mario, Luigi, or Wario, lose a cap in any of the stages which will allow you to do so. Now, while you have no cap on, go to any other level and pick up a star. Return to the castle and enter the main hall and Toad is there wearing your cap. Yes, miracles do happen. Missing Characters in Ending Sequence
It's pretty simple: don't unlock the character from within the game, and you'll notice said character missing from the ending sequence. Square Eggs
Start as Yoshi, then go somewhere with the small square crates, lick it up, swallow and your egg should be a square.