You have to give a guest to one of the following names to activate the corresponding cheat function:
Increase money by $10,000: John D Rockefeller
Increase park value: Andrew Gillett
All rides irresistible to peeps: Jon Roach
All coasters irresistible to peeps: Sam Denney
All rides and coasters to never break down: Frontier
All guests get sick: Make Me Sick
All guests ride every coaster before leaving: Sam Denney
All peeps laugh: Atari
Peep jumps for joy: Chris Sawyer
Peeps dance: Shifty
Big explosion: Atomic
Peeps take photos: PhotoStory
Lots of ducks appear: A Hitchcock
Display frame rate: FPS