Unlimited Time
At the password screen, enter the following medals to acquire unlimited time:

Diamond
Honor
Cross
Crown

Invisibility
To make yourself invisible to Mr. and Mrs. Tweedy, enter the following medals into the Password screen:

Crown
Bronze
Honor
Valor

Skip Stages
Enter the following medals on the password screen:

Honor
Valor
Bronze
Silver

This will allow you to skip to the next stage by hitting the SELECT Button during the pause screen.

Explanation of Medals
In order to enter the codes below, you'll have to understand which medals are which. At the Password screen, you will see a line-up of four medals. Each of them starts out bronze, but can be manipulated to cause different effects. Each time you press down on the D-Pad, you will change the medal. The following is a list of medal names (in order) as you scroll down through the list:

Bronze
Cross
Valor
Silver
Honor
Bravery
Crown
Diam
ond