The secret codes are:
CBAACABC - Costume
BCBACABA - Love Potion
ACBCABAC - Hair Potion
ABBACBCC - Cake
BACBCACB - Steak
To use them go to the shop and press A really fast multiple times until the shop owner looks 'shocked' and then quickly enter the code. You'll know if it works because it shows the item, otherwise try again...it took me a few times to get it to work.
Some say you get something after doing all 5 codes but I've not seen anything yet...