Secrets like:
* Die to the Shy Guy training in Booster Pass, repeatedly
* Go behind Knife Guy to trigger a secret mini-game, win many times to get the Bright Card, then find the specific golden chomp and jump 3 times over it to find Grate Guy's Casino, then play the secret mini-game in there and win 100 times to win the Star Egg
* Equip the B'tub ring to Toadstool and use the Mystery Egg, an otherwise useless item with high sale value, 10 times with only Toadstool to get the Lamb's Lure. Then use that 48 times to get Sheep Attack.
* Lazy Shell questline
* Beetle Mania minigame
* Hidden chests
* Culex
* Marymore servant
* Fireworks ending
* And then all the little minigames that are presented to you throughout the game.