Different ending:
Complete the game in Legendary mode to see a different ending to the game.
Fast Reload:
Press 
then press 
All levels:
Create a new player and name him Micosil to have access to all levels.
Always Invisible:
At any level, press 
and in the menu hold down 
and then press:
God mode, all guns and infinite ammo:
In the main menu hold down the key 
and press:
Complete guide for Halo on Xbox:
Here you have the complete guide by Meristation