Floating objects
From Halopedia, the Halo wiki
In the Halo 2 there are two places where there are floating objects.
Floating Hats
This happens at Cairo Station(level), after the cinematic rolls, as you descend on the stairwell, you will turn and there will be several SMG's and Battle Rifles. Adjacent to this, is a crate where an officer has left their cap. Melee the crate with the Naval Officers cap on it repeatedly. The crate will eventually slip from under the hat, and the hat will be there, motionless. You can't do anything else with it. It's just a minor programming error by Bungie.
Floating Weapons
This is on many levels and to achieve this, find a human crate with weapons on top of it. Then, melee it and watch the weapons float mysteriously! If you whack the weapons again, they'll just fall onto the ground.
