Soccer ball
From Halopedia, the Halo wiki
In the Halo 2 Metropolis (Level), there is a giant soccer ball hidden in the rooftops. There is speculation that it was put in there for Bungie to test the physics engine of the game, and they left it there for us to have fun with.
A red ball similar to the soccer ball is shown in the Limited Edition DVD. There is a section that talks about the special testing level that Bungie uses to test weapons, enemies, vehicles, environmental effects, sounds, etc. One of the scenes portrays a Warthog driving and moving the ball around.
Walkthrough
First Method
To get it, retrieve the Scarab Gun and get back into the Banshee. Then head along the highway after the amphitheater until you find a building with a huge chunk taken out of it. At the bottom of the hole you will see the giant soccer ball If you manage to get it out before the Scarab comes by, it's really fun to see the Scarab kicking the ball around on its way to the canal. The Scarab Gun is also good for pushing it around.
Second method
There is actually another uncommon way to get the ball. If you are not able to get the banshee to that part of the map, don't fret, all you need is to be in co-op and have a Wraith. Position one person at a certain position below a smaller building, but one still missing a chunk (Note: You will see it as soon as you walk back out into the street.), and the second person in the Wraith, with both people facing each other. Have the Wraith boost. If the person not in the wraith jumps at the right time he will be boosted up into the broken part of the building. You are so close, but yet so far. Look around and you will see two metal things sticking out, one on each side. Jump onto the lower one, and after doing this, jump onto the second one. After doing so, jump up and on top of the building. Walk around until you see a raised area with a fence type thing on it. It is missing a part an the wall, so jump up and through the hole and walk up to the building mentioned in the paragraph above and walk into it to get the ball. Just punch it and move it until you knock it off the building and onto the ground. Get yourself down and into a second Wraith and you and your friend can play soccer with the ball.... and wraiths.
Third method
There is also one more way of getting the soccer ball down from the roof tops. If you know where the ball is located and just don't know how to get up there, follow these steps. When you come to the part when you can hit the Jackal in the back and steal his beam rifle, look for the gauss hog and go through that area, in the next part stay in the hog until your able to kill everything, but leave a ghost. Get in the ghost and ride up to the part where you recon with the marines (and eventually jump onto the scarab). Make sure you jump onto the Wraith and beat down the driver. Get into the Wraith and drive to the area with the ball. Fire Wraith shots into the area. This takes a very long time but it works. (This may require the Sputnik Skull; For I had the Sputnik skull when I did it, but I am not entirely sure. It would be much more wise to try the other two ways first.)
Fourth method
For this method, Sputnik is highly recommended. In Metropolis after the waterfall part (the entrance is guarded by a wraith and 2 Jackal Jackal snipers) there is a big building with a large piece of broken glass in front of the entrance you just exited. Jump onto the ledge and grenade jump into the big glass crack. From there navigate up the support beams, you need to grenade jump at least once to do it. Once on the top support beam you can exit the roof. From there get to the ball. Its around the corner with the trees. Melee it off the building if you please (Sputnik is making this relatively easy). If you jump off you'll die so you instead need to backtrack down into the room with the debris, jump down the the small ledge, and onto the ground. From there you can both hijack wraiths and use the wraith boost to play soccer with the Co op player (if you do have one). This is guaranteed to work but you don't get the fun of using the scarab gun to push the ball around.
Fifth method
Also, you can get the Sputnik skull, and then go to Metropolis. At the point where you come past the area with the waterfall, you will see a ledge covered with trees up high, next to the football. Get a Warthog, and make sure it has 3 people in it (including you), otherwise it won't work. Face directly in front of the ledge, on the raised part in the middle of the road. Now throw a grenade at the back wheel, and hop back in. With luck, the Warthog will flip through the air onto the ledge of the half broken building. If it doesn't get into the building, just grenade jump up.
Soccer Ball in Halo 3
While not an easter egg, the Soccer Ball appears on the multiplayer level, Foundry, Standoff and Rat's Nest as a Forge item.
