Fallout 4: the Robotics Disposal Ground GOLDMINE!
When you boot up Fallout 4 for the first time you don't need to look too far past the end of your nose to find some epic loot. The Robotics Disposal Ground is a small junkyard that holds a few special secrets, entirely available from the second players exit Vault 111.
Where Do I Find It?
You can head straight to the Disposal Ground from the Vault by turning to the left when you come upon the bridge across the small river on your way back to Sanctuary, and follow the shoreline of the river, which then becomes a small lake.
An alternate route would be to head to the Red Rocket Truck Stop and following the shoreline on the opposite side of the lake.
Regardless of route choice, the Disposal Ground is at the tip of the lake, as seen on the map below.
Okay, I'm Here. Now What?
Don't head in to the yard just yet! It's absolutely infested with mole rats! Enter the small concrete room to the left; this is where the cool stuff happens. There is a terminal on the far side of the room, already unlocked. When you boot up the computer, launch the holotape and activate the "Combat Sentry Proto MKIV". Before heading out, pick up the multiple military-grade circuit boards (these have circuitry, a fairly rare resource in them) and a stimpack. If you're interested, there's a weapons workbench in the room as well.
Go outside and watch, or participate in, the carnage. The Sentry is not an enemy you'll want to cross. Around six mole rats will bust out of the ground when you go outside, but the Sentry will make quick work of them. (If you're still a low level you'll notice the skull beside the Sentry's name. That means don't touch him or you will die.)
Shoot and Loot!
Dogmeat was feeling photogenic today.
Time to scavenge! Grab the mole rat hides (for the leather) and travel to the far end of the junkyard. To the left of my vision in the photo above, you'll find Flamer fuel (a relatively rare ammo type), though you'll find no flamer. In the green crate behind Dogmeat you'll find some ammo, materials and weapons. There was a fusion core (used to power a suit of Power Armor) lying in the scrap heap as well, though I can't remember its exact location; have a look around.
Now pay attention! Behind the pile of cars beyond the green crate, is an amazingly powerful weapon, though it comes with no ammunition. Climb atop the cars and the Fat Man mini-nuke launcher will be up for grabs. This is one of, if not the best weapon in the game and it's extremely rare to find the launcher and the mini-nuke ammunition in the same locales. For that reason, I recommend you keep it with you, or drop it in some place safe.
Don't worry, there's more!
Blow This Popsicle Stand!
Head back into the concrete room and access the terminal once more. Boot up the holotape and set the Sentry to self-destruct. (Another (foolhardy) option is to take on the Sentry yourself. You will gain a ton of experience if you succeed, but the more likely outcome is your death.) Go outside and stay far, far back from the robot. There is a small explosion, followed by a massive flaming-mushroom-cloud-emitting explosion that's just as likely to kill you as fighting the robot in hand-to-hand combat.
You'll be left with a scene similar to this. Loot that sucker! You'll find two fusion cores... I'm sorry did the people in the back not hear me? TWO FUSION CORES! You'll also receive forty 5.56 rounds, another military-grade circuit board, two biometric scanners (relatively rare), a power relay coil (again, relatively rare), and six pieces of aluminum!
When you're done at the junkyard, cross the street and drop down into the ditch. Turn to your right and you'll see a downed plane in the distance. Just in front of the plane you'll find three pieces of Power Armor: a torso, a right arm, and a left leg.
By the time you reach Concord and gain access to your first suit of Power Armor, the items you've received from the junkyard will help exponentially. You'll have three new fusion cores that'll make your suit last well into Diamond City, extra ammunition for your minigun (which you acquire alongside the Power Armor), one of the best weapons in the game in your arsenal and much more before you hit level 5. This is a spot that you do not want to miss.