Main Easteregg:
There are 6 orbs (blue, yellow or purple) out of bound you need to shoot.
1.In the right eye of the big skull(purple)
2.On top of the lighthouse in the storm thingie(blue)
3.In the middle most crows nest of the right ship(yellow)
4.above Widow´s Wine there is a ghost to the right of that ghost(blue)
all these can be seen from spawn
5.On the cliff above the locked door in spawn. To see this one go a little in front of the machine pistol wall buy(by Widow´s Wine) and look up at that cliff to the right of the ghost (purple)
6.Go to Mule Kick and look at the box spot in the Double tap area, it will be a little higher (blue)

Now a text will appear on screen informing you that you are done. A door near Double Tap will have opened revealing a key, use this key at the door behind the Vending Machine(the one having all the perks), pick up the hammer in that little room and go back to spawn, there is a crate you can now open. The key of this crate allows you to open the door in Spawn but first you will have to find 4 different colored skulls and offer them to thei respective altar.

Blue: At the little Island you reach via jump pad, the altar is to the right of Stamin up.
White: directly by PaP, you need to buy the door and the altar is also right there.
Red: Opposite of Mule kick on a crate, altar to the right of the blue one.
Green: In the spawn areahead toward the big skull and look to your left it will be there on a crate, the altar is to the right of where you found the red skull.

After offering all the Skulls you will be able to open the door in spawn revealing another 250 points door, but behind that door is a teleporter, whichfunctions as a buyable ending and in this teleporter is the key to the Bow (It is just the wolf bow from Der Eisendrache).

Side EEs
There is a three part code you can enter one digit at the time at three different locations.
You will need power fot this and will get a Gobblegum mashine for completion.
1. In spawn Below the Pap mashine under the pier is a clipboard with a Number enter it in the orange symbol right there
2. you will need to buy the first door, but the clipboard is above the Olympia wall buy, enter this digit not at the symbol right there, but at the symbol next to the Bow.
3. The clipboard is above the box spot in the Double Tap area, enter this digit at the Olympia

The altars that function as soul boxes only get you powerups after filling about 15 souls into them.
I think there is no other use for them.

You can build the wunderwaffe DG-2 on the island reachable via jump pad the parts are easy to find except the one in the mouth of the big skull, there is a little one on the rail thingies.

The bow you can only aquire after doing the Main EE you will need to fill up the Dragons and the key at the teleporter.

I think that is all, you can correct me tho.