After watching the bonus footage on the DVD, it will give you two letters. "tr" after the alternate ending, "ea" after the animation bit, "su" after the deleted scenes, as well as "re" after the on location title.
Press your Enter button while one of these hints are shown. They will take you to another screen where you have to order the letters to form the word "treasure", of course.
You now get the screen telling you the secret key: 397. Also, you have unlocked the first couple of easter eggs.
One of these, "Riley Pool's Decode This", asks you to solve another riddle, and gives you two numbers: 04 and 21.
Back on the "Bonus Treasure Hunt" screen, go up until the hidden sign above the pyramide becomes visible. Select, and enter the code 0421.
You will have finally landed on the screen called "Special Viewing Mode", which also gives you the master code: 405.
In the root menu, select the book on the top right corner [because, as stated in the movie, secret maps are always marked in the top right corner]. The new screen asks for a secret key or a master code.
Now, if you enter the master code 405 you get the extended list of bonus features.
All easter eggs were generously submitted by SirDonk