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.

Procedure
You will need a partner to go on this vacation.

At the end of the mission, do not undo the plasma barrier in the room that Johnson is 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 of the butterfly to get up on top of the corner. Once above, kill your partner. Now 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 them to keep walking upwards on top of the rock so they can jump onto the very top roof of the building. Once they are safely up there, kill yourself so you can join them. 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 the makes up the ground when out of the level. 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!