Here's some cool codes to help your game.

Code       Effect
JHAMMO     Access all tracks
CMGARAGE   Extra cars
CMNOHITS   Collision detection off
CMCOPTER   View of helicopter
CMCHUN     Go-kart view
CMSTARS    Starry sky backdrop
CMTOON     Cartoon horizon
CMMICRO    Overhead viewpoint
CMDISCO    Disco fog
CMLOGRAV   Low gravity (affects cones)
CMRAINUP   Rain falls upwards
XBOOSTME   All car speeds doubled
CMFOLLOW   Trackside camera view
CMMAYHEM   Makes other drivers more aggressive
CMHANDY    Big hands on steering wheel