Peep hole in the sorority (naked girl):
Just out of the picture on the right there is a nude girl. If you change the X and Y axis controls you will be able to look far enough over to see her.

Nude mode:
Successfully complete the game then buy 'Everybody Naked!' at the bar. This secret costs 269 secret tokens and can be turned on in the 'Bonus Options' of the 'Extras' section.