Press X, B, Y, X, B, Y, B when you are in a park and if you enter the code correctly you will hear a confirmation noise. When you have heard the confirmation sound press X at the 'Mission Selection' screen to complete the highlighted mission and unlock the next mission. Every time X is pressed on a highlighted mission the rank of the completion will increase from Bronze, to Silver, and finally Gold. Press B at the 'Mission Selection' screen if you want to reverse your progression on a mission and remove the highlighted mission medal.
During the game when you are in any park quickly enter the following codes to unlock the indicated effect. You won't notice anything immediately but if you entered the codes correctly you will get the effect you wanted.
Unlock ALL Parks: Press X, B, Y, X, B, Y, X.
Unlock ALL Rides in Current Park: Press X, B, Y, X, B, Y, Y.
Unlock ALL Mini-Games in Party Play: Press X, B, Y, X, B, Y and Right on the D-Pad.
Unlock 500 Thrill Points: Press B, X, Y, B, X, Y, X.