Ok I was browsing Halopedia tonight and found some awesome stuff in the Trivia section of their level articles. Maybe these little things are old news to you all but after 2,000 games played I had no idea about some of them.
Here's a compilation of the most interesting imo
High Ground
# In the piles of trash, there is a paper crane.
# You can push the bunker hatch back up by deploying a gravity lift inside the bunker.
# There are small fish that can be seen and killed and will wash up on the shore.
# A picture of Cortana appears for a split second on the console closest to the flag.
# The Radio inside the fort where the flag spawns transmits three different Morse Code messages. When translated they are "Frogblast the ventcore" [AWESOME] a reference to Bungie's classic FPS Marathon, "sababwl" an acronym found on the Halo Soundtrack which was a quote from Bungie's Martin O'Donnell back when the original Halo was about to be released, the acronym stands for "shiny and bumpy and bursting with love!" and "Don't make us kick your ass" a classic Bungie saying.
Epitaph
# While keeping silent, players can hear the Halo theme music being played in a deep synthesizer-like tone every few seconds.
# When inspected closely, players can see that the "characters" on the windows of the complex scroll from the bottom to the top.
# If you go outside and look out into the distance at the grey clouds, every now and you can see lightning bolts.
# Upon going outside of the building in forge, you will see a large cloud formation forming far off in the distance. If you look at it from the right angle, the clouds look vaguely like a spartan or marine upper body with the head facing the left, the arms extended and the hands in the "call me" position (thumb up, pinky extended). This is the same type of "egg" seen in Last Resort, although the one here may not be as clear cut as the other.
Isolation
# There is a ghostly face that can be seen when standing at the ghost (vehicle) spawn and looking upwards.
Last Resort
# While on the beach, and looking to the right up in the sky, you'll see a cloud that looks like Master Chief (flexing).
Narrows
# If you look off to the side, you can see another bridge exactly like Narrows. That bridge and this one (Narrows) are probably part of a extremely large cooling installation, as said in the description of the map.
Rat's Nest
# If you listen closely to the big green pipes, you can hear Drones flying and bumping into each other inside the pipes.
# There are Hornets and Pelicans seen flying in the background of the outdoor area. The Hornets can be shot down with a Spartan Laser.
Snowbound
# In the sky, the visible planet has a Marathon symbol on it.
# Beneath the Bungie logo in the screenshot of the battle, you can make out the term Out of Ammo written in the sky. Beneath it is Press (Y) to...
The Pit
Inside the room containing the sword, there is a bright yellow phone with a red error message on it stating that it is out of service. Melee and it will fall on the floor. Then melee it until it is upside down. If you zoom in on it, there is a Windows Logo.