Fun with flatulence:

Doctor Hawkins needs a new diet. At anytime while
playing the Doctor press the following combination
of keys. Left and right trigger, left on the D-Pad,
and A on the key pad. Press all these at the same 
time and watch Doc Hawkins do something "Nasty"

                               
Kurt in boxer shorts:
Hold L + R and press X, X, Y, X at the main menu.
                               

Matrix Mode:

To get the "Matrix Mode" (which is just a slow 
motion mode), play as Max the dog. Then hold the 
R trigger and press Up on the D-Pad four times.

              
Roundabout View:

Press up down left right in this order to create
roundabout view while in the pause menu.