Hamtaro - Rainbow Rescue - Gameboy Advance
Unlimited Acorns:
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Press Up, Down(2), Left, Right at the 'Press Start' screen.
Secret Coloring Picture at the Clubhouse:
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Stand in front of the blackboard in the Clubhouse and press the A button.
After reading the message, press the L, R, A, B buttons in this order and
you'll receive a secret coloring picture.