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
Walking Method
It is possible to "walk" the entire way onto the island, although it would be more reasonable to take the Banshee method. However, you'll need to go through the lake, jump onto a slope protruding from the Control Center, and crouch/grenade jump to some other slopes and ledges. When you reached a platform that touches the island, you can start your "vacation" there. Coming back down is a lot harder, however, since you'll have to jump off ledges and you might die from the height.
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
Fly towards the the Control Center at the highest possible attitude and wait until you reach a large slope with dark carvings on it. On the top of the slope, there is a flat platform. Your Banshee maybe not fly to the top, so you'll have to ditch the Banshee behind and walk toward the platform yourself. If you are a skull-collector, 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.
Ghost Method
There has been a report of someone finding a damaged Ghost left over from the start of the level.
You can do all the things that are possible with a Wraith and Spectre, except at a faster pace.
What to do
You can explore the entire island, as well as going all the way to the top where the Black Eye Skull would be, but not to the top of the Control Room. From here you have a great view of the level.
