In "Assault on the Control Room," you can get a banshee shortly after the tank. Take the sniper rifle and the rocket launcher. Clear the enemies, and then go to the boulder on the right side of the tunnel. Look up and you will see a rock bridge. Now, while still looking up, start moving forward until you see a platform coming out of the wall. Zoom in with the sniper rifle to the center of the platform, and then without moving change to the launcher and fire. A banshee will fall off the ledge, and you can use it.
Easily defeat hunter enemies
Equip the pistol and go behind a Hunter enemy. You'll see an orange spot on their back; hit the orange spot once and the hunter will die.
Hidden area
In the chapter where you must find Capt. Keyes in the swamp, there is a secret area you can access right from the beginning. When Foe Hammer says "The Last transmission from the captain's drop ship was from this area. That was over twelve hours ago." immediately start pressing your grenade throw button. Whatever you do, DON'T MOVE! After a few seconds, the drop ship will take back off, and you'll still be in it. It flies for about 20 seconds then crashes. Exit the drop ship. Now walk directly away from the ship until you reach a clearing. Head towards the center and out of the fog you'll see a lone marine standing there! He won't move or say anything, unless you shoot him in the foot with your pistol.
Hidden music
In assault on the control room near the end of the level on the rock road there will be a banshee steal the banshee and look behind you. If you look up you will see 3 sniper spots a bottom, middle, and top go to the middle one with the banshee land carefully and go to the right side and look at the blue light wait a while and you will hear a hidden track.
Megg
"The Megg" is one of the hardest easter eggs in Halo. After all once you see the Megg, its game over. Start a game on the Pillar of Autumn while on legendary difficulty. Jump on the cryo tube Master Chief comes out of. Jump on the shield recharger thing. Then, jump on the walkway. You'll see some barrels. Jump on the top one. Wait for that guy that woke you up to die, then jump down. Get to the captain and shoot him. As soon as Cortana says, 'rampant', find the door the invincible marines came out of. If you can get past them, another door will open. The Marines can't get you while you're in here. Look up at the 'M' made out of bullets. After you see it you're trapped and the marines now can kill you.
Random messages
Go to create a new account and type in .fortune for a name. As soon as the name is completed, a random message will appear on the screen. Re-enter the name for another message.
Rex in blood
On the last level the Maw, on the last checkpoint, when you go off the big jump onto the small bridge, drive the warthog so a quarter of it is off the bridge. Jump on top of the warthog and jump on top of the wall on either side (the one closest to you) and follow it up to an upper, smaller bridge. There are two hall ways and go in the one with blood in it. Look to the right and you will find the name Rex written in blood. What does it mean ?
Talking grunt
You may need to restart this stage if you pursue this Easter egg. In the last level of Halo: Combat Evolved, you'll arrive at a second grouping of tunnels after the dropship pickup goes away. Take the first tunnel on your right and there should be a fork leading to a dead-end half way through. If you head partly that way, you can hear a grunt talking about something phenomenally droll.