Sierra 117 vacation

On the 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.

Procedure
You will need to do this vacation glitch in 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 nearby where the Iron skull can be found. Face the opposite direction of the skull's location and go into the corner. Now do either the elevator trick or the butterfly 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 break the barrier and respawn on top of the rock. Get him/her to keep walking upwards on top of the rock so he/she can jump onto the very top roof of the building. Once he/she is safely up there, kill yourself so you can join him/her. If you start slipping off, quickly walk towards the rock and jump onto the rock, 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, but you can actually walk on the canopy/grass stuff that makes up the ground when out of the level. The grass makes weird crunchy noises when walked on. It's 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. Have fun!

This glitch is usually called the "Stairway to Heaven", much like the one in Halo 2 in Burial Mounds. It is found by going through the grassy area, walking up the hill, turning around and jumping on the invisible barrier. Though there is a limit to how high it is possible to go, the view is still quite nice.