Delta Halo vacation
From Halopedia, the Halo wiki
The Delta Halo vacation is a small programming oversight in the Halo 2 campaign level Delta Halo. It allows one to go and explore many parts of the level previously thought inaccessible or just scenery. It is similar to the Rooftops Vacation, the Cairo Station vacation, and the Metropolis vacation.
In the very beginning, there is an archway with a turret (closest to you) mounted on top. Walk up onto this rock formation, and to your left there should be the steep hill that leads to the upper plateau of the map. Stay close to the hillside and walk over as far as possible without falling off the cliff. Turn towards the hill and make your way up through trial and error. This method does not require any skulls or grenade jumping, it only requires at least three well-placed jumps to achieve.
First play until you get to the part where you get the tank. Lower the bridge and destroy the Wraith, the Banshees, and any Ghosts that come out of the tunnel. Next back up until you're back in the middle of the bridge facing opposite the bridge control building. Look about 45 degrees to your left. You should see a wall with a lot of growth on it. Drive the tank or Ghost up the wall. This is actually quite easy to do, despite the way it looks. Simply drive your tank towards the wall and don't stop. If you fall off, try going farther to the right. If approached correctly, you should encounter some resistance halfway up the wall, simply drive on through. If you succeed you will be on the other side of the level, overlooking the area with the temple, and the energy shields.
Play until you reach the part where you have to kill the four Jackal Snipers, just before you reach the part where you have to lower the bridge. Using any means necessary, get yourself inside a ghost. Proceed backwards until you almost reach the Jackal Snipers, where the lane curves, and there is an indent in the wall with a large rock. Drive slowly at the wall about one meter to the right of the rock, and your ghost should aim upwards. Boost while zigzagging slightly left and right until you reach the top. This saves you the time taken to fight down the bottom to lower the bridge.
When driving the tank, take it to the first outside space after the Bridge, on the right is part of a cliff that slopes partially downwards. In some parts, it is possible to drive the tank up. On the ledge there is also a dead ODST next to his HEV, the body has useful weaponry. Then drive the tank up the cliff side near to the ODP. You should be able to get above it. At the top you can go anywhere, even up the mountains in the background. However, going back down into the next area will likely kill you, and reaching the door to the hologram room is inaccessible.
At almost any point in the level, one could use the Sputnik and Feather Skulls (An MCC exclusive skull.) to jump over cliff edges and break the boundaries. This relatively easy, although the height of a jump may accidentally cause a suicide if one does not land correctly.
What You Can Do
You can explore almost any part of the level from here. You can climb all over the mountains, including the tallest of them all, and get a great view from the top of the level. You can drive around, and go into the lake behind the temple (called Lake Puddles by some) and have a swim. You can go down to the other lake, in front of the temple (Lake Regret) and see that Regret's Temple actually floats. And with Sputnik and Feather, you can fly and climb up to Regret's temple itself, with much less detail, of course. You can also see the Sentinel wall. You can even skip to the very end of the level (however, the Honor Guard Elites will not spawn, so you cannot complete the level). Many other things are probably possible.
Also, if you drive your Ghost onto some beaches or near vertical cliffs next to lakes (such as the beach/cliff under where you start) you will die, as these are areas where you are programmed to "fall to your death" like the fall to your death glitch on The Great Journey. Also, you can find a room with two dead Grunts and two Energy Swords near the ruins.
You can also find a mysterious switch beyond the temple which has an unknown function.
However, you should be somewhat careful. If you go over a hill and go flying through the air, even though you could still make contact with the ground, long jumps will kill you. Stay on solid ground.