There is actually no really built in cheats, but there are some things to help your park.

Name a guest one of the following names

Chris Sawyer-Takes pictures of everything
Simon Foster-Walks around painting pictures
Melanie Warn-Makes guests completely happy
Katie Brayshaw-Waves at everyone she meets
John Wardley-Says "Wow!" a lot
John Mace-Gets on rides for free
Damon Hill-Goes extra fast on go karts