At the start screen and hold L+R and press Up,Up,Down,Up,Down,Y,Start

Jump from any height
Go into combat mode before you hit the ground and you can jump from any height without hurting yourself.

Avoid falling damage:
Press R as before you hit the ground to enter combat mode. You will not get any damage from falling.

View art gallery
Find all artifacts in the game.

Pistol whip:
Press X button to pistol whip your adversary. You can do this with any gun.

Drink from canteen:
To drink from the canteen, hold A. If you instead tap A, it will take longer. Holding A also helps when you fill your canteen.

Skip all poachers:
In the level where you start out in a room enclosed by a vine door, cut the door and walk forward. It should start a short intermission sequence of Indy running towards a little window. Skip through it and when it is over, select the whip and jump. While in the air press A. If done correctly, you should now be swinging above a courtyard. Swing to the other side and continue the mission.

Missing hat:
Pause game play and look carefully at Indy to notice that his hat is gone. Look at his forehead during game play, then and look back at his forehead.