Cheat mode
Press [Esc] prior to you starting a mission and enter the "General Options" menu. Then, enable the "Tour Invulnerable" or "Unlimited Ammo" options. Note: This will also result in less points, no bonus points, and no medals in the game.

Every mission and medal
Create a new pilot and enter FongFong as it's name. Then start the game, then immediately exit. Now enter the "\lucasarts\xwingalliance\" folder and delete the "fongfong0.plt" file. Start the game once again. Then every mission, intermission sequence, and medal will be available.

Hidden picture
After you have played a combat simulator skirmish or combat engagement, a screen will appear with your score and those of any other players. When your at this screen, press [Shift] + [Ctrl] + [Alt] to see a hidden picture.