During gameplay hold R1 + R2 and enter the following codes to unlock the corresponding effect.

Get 9,999 Gopher Bucks:
X, X, Square, Square, Sqaure, Square, Triangle, Sqaure.

Get ALL Knapsack and Flower Pack Items:
X, X, Triangle, O, Square, Square, Square, O, Triangle.

For an easy way to get some extra coins, at the point where you are told by Ben to use the metal detector to find Gopher's coins, go to the barnyard and on the right side farmer's house there are some yellow flowers.

If you use the metal detector there, you should find some coins, at night keep leaving and rejoining the party and go back to that same palce to find more and more coins.