From the options screen, go to the Hats section and select 'Enter your code', then input the following codes to unlock all of the secret hats for Snoopy.

Hat	How to unlock
Chef Hat	Charlie Brown, Pigpen, Linus, Lucy.
Football Helmet	Sally, Schrodinger, Snoopy, Marcy.
Santa Hat	Woodstock, Charlie Brown, Pigpen, Linus.
Top Hat	Lucy, Sally, Schrodinger, Snoopy.
Visor	Marcy, Woodstock, Charlie Brown, Pigpen.
Winter Hat	Linus, Lucy, Sally, Schrodinger.