Riding a Pelican
From Halopedia, the Halo wiki
On some levels it's possible for a player to climb or jump on top of a Pelican and ride the ship to its destination.[1]
Walk Throughs
Metropolis
Play through the Metropolis (Level) until you get to the part where you're in a Gauss Warthog with another Warthog riding alongside you (your situation may be slightly different, but that's what is meant to happen). You come into a street with a couple of Jackal snipers, two Wraiths, and a couple of Ghosts. Another Wraith and two Phantoms come momentarily, followed by two Pelicans.
Notice where the Jackal snipers were standing: raised areas between the streets. Go to the far one. This is the point where the Pelican will drop off some marines for you. Be at the top of this area before the Pelican begins its descent.
As soon as the marines have exited the Pelican, jump toward the passenger bay and hold the X button. I suggest aiming for the bottom of the Pelican, right near the back entrance. This should allow you to climb in the cargo bay and go for a ride.
After a short trip, the Pelican falls to the bottom of the level. In Halo: Combat Evolved, a free-falling Pelican would travel the exact same path every time, but things might be different here. When I did this in single-player mode, two marines, the Pelican, and I all landed safely at the very bottom of the level. In the three times I've tried it in co-op mode, though, everybody landed on a giant concrete platform (too far down to get back into the level, and too far up to continue to the very bottom). The Pelican continued its way down, while we were stuck there.
If you manage to get to the bottom (blue area), you notice two pretty interesting things. The first is that the Pelican lands facing up, or keeps rocking back and forth until it looks straight up. The second is that when you walk up to it, it says, "Press X to flip Banshee." Banshee, huh? Yeah… Then if you press X it flips end for end and if you time it right it will hit you and send you flying into the air (I managed to get the the scarab gun once doing that!!) Also if you are lucky you can get the Pelican to end up as though it had landed perfectly, you can get into it and go all the way through to the pilots seat and press X. You can now fly the Pelican as though it were a Banshee. It looks prety cool having a pelican doing barrel rolls and flips!!
Outskirts
Also, if you jump onto the rooftops at the beginning of the Outskirts (Level) and completly blow off the battle in the first part, a checkpoint will appear and Sergeant Johnson will spawn and begin boarding a Pelican which will also come. If you jump onto the Pelican it will fly off with you on it. You won't get far though because it disappears in mid-air!
Delta Halo
Another example is on the level Delta Halo, where you continue as normal until the pelican comes in and drops off the Warthog. Get to where the Shade turret on a small hill was and then wait for the Pelican to come to a halt. Then Grenade jump up to the Pelican. It will fly around with you on its back and then move forward faster making you drop down to your death
