World Championship Poker

Find $200
Look at the cola machine in the Air Hockey room, then walk back. Press A + B + R immediately.
(Thanks to Bear356)

Find $200
Holding the Right-shoulder button down, walk in and out of the southern-most casino's loading point. Walk back and forth to find money.
(Thanks to Bear356)

Unlock Casinos
To unlock all the casionos, at the "TOUCH SCREEN TO START", press Y, X, Y, B, L, R, then control pad. Left for the Amazon, right for the Nebula, down for everything up to Renaissance. If done correctly, you should hear chips.
(Thanks to Bear356)