Different ending: You can see a different enging to the game if you can complete it with more than 70 Red Gems.
Level select: At the title screen go to options, then enter the following sequence using the second controller:
L, R, Start, Select, X, Y, A, B. If you have done it right a sound will confirm the cheat has been activated. And you'll be able to select which level you want to play when you go back to the title screen. Hold L + R and press Up or Down to select the level, and Left or Right to select the section.
Extra lives: When you are playing the bonus levels, try pressing the B button when the genie blinks and you'll get an extra life.