Great Journey vacation
From Halopedia, the Halo wiki
The Great Journey Vacation is similar to the Delta Halo Vacation. It allows the player to walk all over and around the Control Room island and in the water surrounding the island.
How to Get There
Wraith Method
Towards the end of the level, instead of going into the Control Room, hijack a Wraith. Then drive through the water and to one side of the island and use the boost to help you get up on the sides.
Banshee Method
Follow the instructions for the Black Eye skull on whatever difficulty level you prefer, not necessarily Legendary, and then you can explore the entire island as you would be able to with the Wraith.
Spectre in control room
Trust me, these steps aren't worth the effort. You can drive the Spectre into the control room, pick up some elites (and sometimes even the sergeant) into it. (NOTE the sergeant will disappear if you jump on the platform.)
What to do
You can explore the entire island, as well as going all the way to the top, but not to the top of the Control Room. From here you have a great view of the entire level.
