Sierra 117 vacation
From Halopedia, the Halo wiki
On the Halo 3 Campaign level Sierra 117, it is possible to take a "vacation", much like the numerous vacations one can take in Halo 2 and Halo: Combat Evolved.
To do this glitch, you will need to play on Co-Op.
At the end of the mission, do not deactivate the barrier in the room that Johnson and his Marines are being held in. Instead, bring your partner to the part of the building near where the Iron skull can be found.
Face the opposite direction of the skull's location, and go into the corner. Now, perform either the Elevator glitch, Butterfly Glitch, or by Brute Jumping to get up on top of the corner. Once above, kill your partner. Then quickly crouch against the rock next to you on the cliff. It may take a few tries, but your partner should eventually respawn on top of the rock, past the invisible wall.
Get your companion to keep walking up the rock so they can jump onto the highest part of the building's roof. Once they're safely up there, kill yourself so you can respawn with them. If you start slipping off, quickly walk towards the rock and jump onto it, walk up it, and jump back onto the roof and you should be fine.
Now continue along the roof until you walk into the trees. It may look like you will fall through to your death, but you can actually walk on the canopy/grass that makes up the ground when out of the level. The grass makes weird crunchy noises when walked on. It is very serene and large. You can explore as long as you want throughout the top of the level around the base where the Pelican picks you up.
This glitch is usually called the "Stairway to Heaven", much like the one on the Halo 2 level, Burial Mounds. It is found by going through the grassy area, walking up the hill, turning around and jumping on the invisible barrier. However, there is a limit to how high you can go, though the view is still quite nice.
There is a second vacation in this level. Go to the Theater lobby, load up a saved film of the level, and perform the Pan Cam glitch.
At the start of this level, in the crater left by the Chief's landing, press Y to switch to Pan-Cam Mode, and then press up on the D-Pad, holding the button until your speed raises to around 60000000.00. Now press RT; you should rapidly fly out of the boundaries and into a white area with green landscape underneath. If that happened, then press down on the D-Pad to reduce your speed to the normal camera speed of 1000.00. At this point, you can explore the first part of this level (from the beginning up to the cave with flares) with the camera placed outside of the map. You can even explore the giant lake.
Also take note that if you move the camera back into the level boundaries, you will have to repeat this glitch to re-exit them, so stay out of the boundaries -- keeping the camera above the first waterfall will work.
There is a third vacation from this level. It requires more jumping skills than the first though. When you get to the part with the sleeping Grunts, kill every enemy. After that is done, go toward the next area, but when the game lags for a second, go back towards where you came from. If you see a big boulder replacing the cave exit, it means you crossed the partial loading point and de-loaded the invisible wall. Now, go ahead and jump next to the boulder, jump onto the ledge next to it, and then jump onto the ramp like structure made of rock in front of it. Climb to the top of that. Now, jump across to the rocks in front of you. If you are still being held back by an invisible wall, try the jump again. This vacation can be used as a shortcut for the level.