Great Journey vacation
From Halopedia, the Halo wiki
This is a very short vacation. Play "The Great Journey" until you get the banshee. Continue until you get to the control chamber door. Instead of ditching your banshee go in, there will be a huge wall, fly OVER it. Get into the part where brutes attack you, and kill the brutes with your banshee’s fuel rod cannon. You don’t have to kill the Brutes, you can just go in the door. Once you get in the door, the screen will turn white and it will show a cut scene. After it is over, you will see a bunch of brutes standing in front of you, turn around, go to the ledge, and jump over it where you will find your banshee. Have fun!
What to do
You can fly around the control chamber with your banshee and also go to the other back door. The back door is closed but you can probably find a way to open or go through it. Also, you can kill Tartarus (brute chief) very easily. Just go over to him and wait till Sg. Johnson takes away his shield with his beam rifle. Then keep shooting him with your Banshee. Repeat this method until he dies. You can also make Tartarus (brute chief) jump off the edge of the platform and fall off. This will not kill him, but can be funny.
Also, if you don’t go into the control chamber, than you can go behind the cliffs on the left and you can get to an entrance into the other base, then you can do crazy things with your banshee.