Flying Hat

Summary
On the level Cairo Station directly after the cut scene you are on a glass platform. Go down the stairs and there is a box with Sergeant Johnson's hat on it. If you melee the box away then the hat will still be where it was, but now it's floating!

You can make a rocket launcher ammo stand straight too. On the Metropolis level go all the way to the end and you will find lots of rocket launchers and their ammo. one spot, quite near the second and third bridge there are a few rocket launchers,a lying down ammo and two Battle Rifles. Melee the two boxes away and the ammo will look like its standing perfectly straight.