The Delta Halo Vacation is a small programming oversight in the Delta Halo (Level). It allows one to go and explore many parts of the level previously thought inaccessible or just scenery. It is similiar to the Rooftops Vacation, the Cairo Station Vacation, and the Metropolitan Vacation.
How to get there
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 on 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 sheilds.
What you can do
You can explore almost any part of the level from here. You can climb all over the mountains, 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. You can also see the flood wall, and be really mean to your friend and ditch him in the middle of the lake! Fun. 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.