Great Journey unused objects
From Halopedia, the Halo wiki
When you have one, proceed to the area in which Johnson blows open the Control Room door and proceed to drive directly into the water. Head towards the island with the Control Room in it. When you reach that island, look for a sloping rock face. Climb it using the Spectre or Wraith, until you are above the water and on dry land. Continue to drive up the island using the boost and the hand brake, when necessary. Then go towards the highest point, either the top of the island's mountain, or the top of the Control Room. Both can be reached by simply boosting and hand braking to the top. When you reach the summit, or the top of the Control Room you can enjoy the view.
But there is also something else you can do. Get out the Beam Rifle. From your high vantage point it is easier to see this. Look to the right, then zoom in to x5. In x5 zoom search for something that looks out of place. When you find it in the sky, to the right of you, zoom in to x10. What you are seeing are the pillars from the levels High Charity and Gravemind. They are likely used for the after credits cutscene, which takes place on High Charity and features a shot of these pillars.
You can also gain a view of these objects, although a limited view, by simply staying in the Banshee and flying to the right of the last cliff on the left (you may still have to destroy the turrets off the Phantom). Your view will not be as great as scoped in with the Rifle, but you can still see the objects.