To enter any of the following codes, you must go to the Mode menu and press L + R.

100 Gems:
Enter V1S10NS (Keep in mind that the 1s and 0 in V1S10NS are really Ones and a Zero).

Sheep enemies:
Enter SH33P.

Orange game:
Enter SP4RX.

Orange Spyro:
Enter SPYR0 (The 0 is a Zero).

Crash Party USA:
Turn your GBA off. Hold L and R and turn your GBA back on.

Thanks to Revolution readers Might Duck, Randy, and Sarah Flower!