Type WAVYGRAVY at any time during gameplay to get a weird special effect.

Type X at the main menu for the obligatory Ambrosia quote-or-poem easter egg.

Type Command-Shift-+ to make Harry a lot bigger than usual.

In Level 3, if you can get past the fans near the start (HINT: CAFFIENE), you can get the green key, which lets you into the Swivel Tournament area... with great rewards!