The Covenant Vacation
From Halopedia, the Halo wiki
It is possible to hypothetically get out of the Covenant level.
You may or may not start from Rally Point Bravo to do this. First you drive all the way until you get the loading done message. Then hijack a ghost from one of the grunts and turn right and drive up the mountain. Once on top, if you keep driving straight, you will be sent out of the map. You can also do this with a Hornet. You will need to be playing on co-op with at least one partner to do this.
What To Do?
If you choose to use the Hornet to get out, you will be able to explore areas you have never gone before. You can return to the first tower to and find the misplaced switch. You can visit a few empty terminals. You can find a secret room inside of the 3rd tower. Or you can return to the 3rd tower's control room. It is also possible to enter the second tower, though nothing is there. There is so much to explore when the barriers are not holding you back as they normally would.
Using the other methods, if you manage to go back to where you started, you will not be able to finish the level. However, using a method similar to the first, you can manage to get back to where you started and you can continue to finish the level. However, at first the level will seem a bit strange until after you reach a cutscene.
When you get to the first tower, do not enter the structure. Instead, kill the enemies outside the structure to make the process easier. Destroy the Shade Turret also. Position your Type-32 Ghost directly between the wall and the base of the Shade Turret. Once ready, boost up and aim for the roof of the tower. This may take a few tries however. (Note: Holding down the Jump button whenever you're trying to get onto the tower makes this much easier.)
After getting on the roof, continue using the Ghost to climb up the ramp until you reach an invisible wall, preventing the Ghost from moving any further. Get off the Ghost and use it as a base to jump onto the top of the wall. Proceed up the ramp and jump onto the snowy edge. If you fall, do not fret as you will be floating mid air. Once you got on the mountain/hill, you are now freely and officially out of the map.
What To Do?
There are limited things you could do in this Vacation. One of them is skipping the whole level till the end. Once on the mountain/hill, proceed towards the end of the tower structure. At the side of the structure, have one player going down the hill. If the player manages to stay alive, the other player would be able to go down the same way.
Be warned, this method is advised to be practiced only in Co-Op play, as failing to do so may result in repeating the whole process again. (If done in Co-Op, any mistakes can be fixed as the player who dies will spawn at the same location as the other player) Be warned, doing this Vacation will result in being stuck in the level as no enemies will respawn in the level.
When battling the two Scarabs, go onto one of them and make it "stunned" by taking out the legs. Once you've done that, get on the legs and stand at the "knee" of the Scarab. Wait until the Scarab recovers from the "stun" and it will start moving its legs, knocking the player randomly anywhere in the air. If the player is lucky and alive, they will be on a mountain or another structure although this is rare. Another method is to use your hornet and quickly get to the Scarab on the left that "roars" and lifts up. To not get automatically killed, jump off of your Hornet as soon as it gets to its highest point. Then rush to the back and blow it up. You will be blown usually towards the mountains and theres a better chance of surviving if you have the invincibility power-up so you do not automatically lose your shields. Most players doing this act would die instantaneously, flying in the air and passing every invisible wall, resulting to what some players call, stretchy Spartan. This could also be considered as a vacation although you will not be able to venture beyond it.
Right before you battle the Scarabs, get in your Hornet. Fly to the right of you, and look up and there should be a bridge. Fly up and under the bridge until you hit a game shield, then fly left along the game shield until you get stuck. Get out of your Hornet and you should be able to stand and walk all over the map now.
Make sure you have a Brute Shot for this. When you are trading fire in a battle against the Scarab, get onto the one on the right. Enter the power core and blast it once with a Brute Shot slug. This should lower its energy shield. Once done, melee it once more and the Scarab will despawn, sending everyone on board flying out of the map. Note: You will die after a while of flying because of a death barrier and confines.