From Halopedia, the Halo wiki
The Rooftops Vacation, or simply The Rooftops, can refer to one of two small programming oversights, similar to The Delta Halo Vacation, as well as numerous others. This oversight allows you reach areas of the Outskirts level of Halo 2 previously thought inaccessible and even skip parts of the level.
At the start of the Halo 2 campaign level Outskirts, several wooden logs should be visible hanging out of the rubble on the wall next to the crashed Pelican. In Co-operative mode, position one player underneath the logs. The second player will be able to stand on top of the first and jump off them to reach the logs. This may take several attempts. You will be able to walk across the rooftops to previously-inaccessible areas, and even to the edge of the map. You can also jump down to where the Pelican drops down to pick up Johnson. The pelican will land as normal and wait for Johnson to get on it. It can be jumped onto solo, but most of the time it will take a partner to jump on it. You can keep up with the flying Pelican by running across the rooftops but it will disappear after ten seconds.
After the Jackal snipers, get a vehicle for a height bonus to jump or crouch jump onto a rusted metal shade. You must crouch because you are too tall and will be crushed by another shade. If you don't crouch after 30 seconds you will die. Jump onto the rooftops to do this vacation.
At the beginning of the level proceed as would to get the Blind Skull. Jump over the hole you just came up from and you will enter a small alley. At the end of this alley is a boundary. stand next to it and crouch jump onto the ledge overlooking the crashed Pelican. It should take a few tries. (Note: do NOT grenade jump. It may blow you onto the crash site). You can cross the beams and explore the rooftops that way or simply turn right and then right again to skirt the courtyard where the marines are. you can go pretty much anywhere after that.
Pelican Jump Vacation
With the Sputnik skull on, wait on the corner overlooking the spawn point for the pelican Sgt. Johnson gets on. As soon as the pelican spawns, jump out towards the pelican. (It should be flying towards you.) If timed right, the pelican will hit you, propelling you up to a pair of cables far above the city.
Play through the first part of the level until you get into the courtyard. Go towards the right and up a bit, until you get to a truck lying on its side. Not very far past this truck is a door to the right. Through this doorway and then to the left is a platform low enough to crouch-jump onto. Go forward and do the same, and then again onto the next rooftop. Turn right and crouch-jump again, then up again, then across the gap onto the next building. Follow the rooftop up ahead and to the left, then jump down and stand on the corner. This triggers the Pelican to spawn, and you can then jump down onto the highway section to your left, and from there onto the Pelican. If you stand on the Pelican's right wing, from the left as you see it, when the Pelican takes off again, you can jump to the roof across from where you started. If you follow the Pelican a bit and then turn right, you will see the Hotel Zanzibar. You can either jump down here and simply skip part of the level, or you can continue on to explore or get a sniper rifle.
Alternatively, instead of going to the courtyard, you can jump onto a light fixture and then up onto the roof as if to get the Blind skull. Through grenades and possibly playing co-op and standing on your partners head, you can go around this entire section of the map, even on top of Hotel Zanzibar. It is actually quite easy to get around with practice. There are some isolated spots, such as at the top of some towers, that you can't get to without spectacular grenade jumping, but there certainly is much to see, except on the far side of Hotel Zanzibar, which simply forces you to jump down. You do NOT need the Sputnik skull to achieve this.
Another way is near the courtyard the is an alley of sorts. go into the corner with the dirt pile, grenade jump to the ledge above. At the end of the ledge launch another grenade and use it to get to the next ledge.
If you're looking for a more straight-away approach with no time wasted, play on co-op, jump on the overturned Pelican at the beginning of the map, jump on your partners head, and follow through on one of the protruding steel beams to after the first walkthroughs Pelican wing area. This is a much more direct shortcut, although testing has not been extensive enough to know whether or not it spawns the enemies and allies at the Hotel Zanzibar and lets you follow through.
Once the Hunters break down the door, kill them in whichever way you see fit (although before you kill them, an interesting glitch will occur if you stand on their heads. Depending on where on their heads you stand, they will vibrate and/or spin in circles. This should only be attempted on co-op, so you can cause them both to glitch out, so the other can't kill you) . Then go through the door and jump on the green boxes, and then onto the raised concrete section. Jump onto the gray bin and crouch jump onto the light pole. Turn left and jump onto the beige/orange colored building. Turn right and go onto the steel bridge and turn right onto the broken freeway. Go to the edge near the rooftops and jump onto the little ledge. Look up a little bit and crouch jump forward onto the roof. From there, turn right and walk down to the end and jump on the other rooftop. Go forward until you pass the tall tower and jump down to the lower part of the building. Go straight and jump onto the heaters (make sure you are on the second row of them). From there jump on top of the small room in front of you and go through the shattered roof. There is an Energy Sword lying on the floor of the room. Written to the left of the sword is the word REX in stones.
There is another vacation in outskirts. Here is how to do it: After going through the Hotel Zanzibar, get in the Warthog and drive to the smashed tunnel. It is recommend that you destroy the Phantom's turrets to make it easier to get on top of the tunnel. Now, see where the Phantom hovers and drive the warthog against the small ledge closest to the opening. Jump onto the warthog and onto the ledge. Now face into the tunnel, towards where the Ghosts are parked, and you should see a small ledge small enough for you to jump onto the roof of the tunnel with.
However. There is an easier way: After going through Hotel Zanzibar you will have three pieces of beach. In the second piece is a small building with two ladders inside. After you've killed all the enemies on the second beach, climb up the ladder and blow the box up. Then you must grenade jump onto the cable (NOTE: You MUST activate the Sputnik Skull first before jumping on the cable). Then, walk the cable until it stops and then you can jump onto the building. Here is a video of it:
These are two alternative methods, but they come up later in the level, so use these if you want the absolute easiest way, as long as you can make it through the level. However, if you want to go on "vacation" as soon as possible, a majority of players recommend "The easiest way to skip half of the level, Outskirts", as this is the other easiest method.
The first method is to play through the level until you get to the part where you have to fight the Drones and the Jackals that snipe at you. Defeat all of this enemies, and then go on the platform with the stairs. Climb up the middle of the stairs, until the wall stops you. Turn left so you're looking down a long alley. Go to the edge of the platform you're on. If you look a little to the right, you'll see that there is a corner. Go to the very edge of that corner. Now look toward the alley, and you'll see what looks like a rusty metal thing that is above a door. Walk back a little, then run and jump at the corner's edge, landing on the brown thing. If you're still looking down the alley, you'll see a concrete beam in front of you. Jump on that. Now turn left 90 degrees, and hold down crouch. Walk forward until you can stand up again. If you're still on the beam, jump on the roof directly in front of you. There, you're on the rooftop!
The other method is to play through the whole level until you get to where you have to fight the Sniper Jackals and Drones, and kill them all. Then walk straight down the aforementioned alley, on the path, and stop at the end of the path, with the white truck on your right. To your left, you should see two green crates. Jump on them, and turn towards the white building with the low rooftops that is right next to you. Jump on the roof, if you land on the lower one, just jump again. To your left is a hole. If you go in it, you can kill two Grunts and get a good vantage point on the small battle going on. If look to your right, you'll see a building. Jump on the part closest to you, then on the gray box. Stop here and look to your left a little. You should see four gray things sticking out of the edge of the building. Jump on the rooftop, and then stand on the farthest gray thing, the one closest to the wall. If you look back towards the rooftop you jumped on from the crates, you'll see a higher roof. Run forwards and crouch jump onto the building. If you make it, you can go pretty much anywhere you want on the area of the level.This is a fun vacation but it is the smallest vacation.