Tomb Of The Ancients: Hidden Area
When you get to the last area in Tomb of the Ancients, go to the dark spot on the third panel which is behind the "Technical Check-Up" door. Press B, Y, D-Pad Right, D-Pad Left, D-Pad Right, D-Pad Left.
If you were in the right place when you entered the code, a crate will appear that allows you access to a hole in the wall. Through the hole, you can view various secrets about the game.
Bonus Stage Unlock
Enter this code before it says bonus stage denied and have it be accepted: A, B, X, Y, X, Y.