The Rooftops Vacation, or simply, the rooftops, is a small programming oversight, similiar to The Delta Halo Vacation, The Metropolitan Vacation, and The Cairo Station Vacation. it allows you to reach areas of a level previously thought to be inaccessible.
How to reach it
Enter the courtyard and kill all the enemies until the hunters come out. Kill them as well. Then go through the tunnel-type thing the hunters made, but do not continue through. Instead jump on top of the protruding wall. Then crouch jump onto the wooden support beam overhead. You should see three pipes coming out of the wall and into the ground, of progressingly larger heights. Jump onto the bottom one an crouch jump onto the remaining two, and then the actual roof. Now you can roam around this part of the level, and find a bunch of neat things to explore. I found a sniper rifle inside a little room once. It is suspected that you can reach the next part of the level this way, with the sputnik skull, but it hasn't been tried yet.
on outskirts when you enter the courtyard run past the jackels and on your right there should be a small gap betwwen buildings. Go through it and it should look empty but if you look where the grunt picture is on the wall there is a ledge above it that you can crouch-jump onto. once on run and crouch jump to the top of the buildin and you on top of outskirts and you can stay on top untik you reach hotel zanzibar.
Alternatively: Right at the beginning of the level, before you kill the first grunt and right after leaving the crashed Pelican site, jump onto the light fixture across from the door, turn left and crouch-jump onto the roof, as if you were going to the Blind skull, except turn around and jump across the gap away from the skull and with the crash site now on your right. Grenade jump onto the ledge to your right and now you can explore all the rooftops in the city.
Different Vacation in the second part of Outskirts
There is another vacation in outskirts that was found recently. Here is how to do it. After going through the hotel Zanzibar, get in the warthog and drive to the smashed Tunnel. I recommend destroying the Phantoms 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. Have Fun!.