When you meet the scientist for the very first time, stand on the mushroom and face him. When facing him, press R, R, UP-STICK, DOWN-STICK, UP-PAD, DOWN-PAD, C-UP, C-DOWN, C-LEFT, C-RIGHT, C-UP, C-UP, C-LEFT, C-LEFT, C-RIGHT, C-RIGHT, C-UP, C-UP, C-DOWN, C-DOWN, UP-STICK, DOWN-STICK, LEFT-STICK, RIGHT-STICK, START. Then if you do it correctly in about 30-45 seconds, the scientist should say, "impressive, you have all the items." Then he should give you all his items, the pressurized fish bowl, the pee shooter, the gliding bowtie, the helicopter hat, and so on.
Beat the game with a controller pak inserted. When you start a new game your energy will be the same as when you beat the game.