As unlikely as it seemed at first, Constantine did, indeed, have a number of nods to the DC Comics Universe.
We probably missed a bunch -- that library of Jasper's was fairly packed and we could only actually see a very small number of them -- but here's what we got.
Let us know in the comments if you saw something we didn't -- like, perhaps, where we can spot the Medusa Mask, which we still haven't seen even though a number of commenters assure us it's there.
The Helmet of Nabu
The helmet of Doctor Fate, formally the Helmet of Nabu, is a golden helmet enchanted by Nabu which can bond with a human host to form a powerful magical being. The most common form this being takes is Doctor Fate, one of the DC Universe's most powerful mystics.
Literally in the same shot as the Helmet of Fate, you can see the Ibistick. Since Ibis isn't nearly as recognizable as Fate, here's a quick refresher:
According to the DC Wiki, "The Ibistick is a magical wand with the Ibis symbol at the top. It is capable of almost any feat when used for good purposes. It would glow in presence of evil and would be ineffective against most black magic."
Ibis, like Fate, is a magical being who inhabits the bodies of ordinary people in the DC Universe.
Ibis began his life as Amentep, an ancient Egyptian price who was in love with Princess Taia of Thebes. As a young man, Amentep had been given the Ibistick by the Egyptian god Thoth, who empowers the talisman. Amentep's throne was eventually usurped by the evil Black Pharaoh, who shot Taia with a poisoned arrow. Using the Ibistick, Amentep placed his beloved in suspended animation to allow her to heal and then cast a similar spell upon himself, hoping to be present when Taia revived.
Centuries later, the mummy of Amentep returned to life in an American museum in 1940, the work of the wizard Shazam. Calling himself Ibis, Amentep set out in search of Taia, eventually finding her at another museum. Seeking to adjust to this new world, Ibis used his vast powers to become a crimefighter, operating primarily in Fawcett City and working with the Marvel Family on Earth-S in the pre-Crisis on Infinite Earths DC Universe.
The most current iteration of the character is Daniel Khalifa, an Egyptian-American teenager who is chosen by Amentep to succeed him.
In addition to the Marvel Family, Black Adam and Isis, Amentep and Ibis are connected to other supernatural DC characters like Zatanna and Black Alice.
With The Flash building to a Crisis on Infinite Earths-style Red Skies Crisis, it's perhaps not surprising the Constantine features Pandora's box, the MacGuffin at the center of the Trinity War event and the defining equipment of Pandora, the mysterious woman who was present at the rebirth of the DC Universe at the end of Flashpoint.
Shaped like a small skull and fairly distinct-looking, the box opened up a gateway to an alternate earth and unleashed evil on the world...well, Forever Evil if you want to get technical about it.
Is Liv's father Baron Winters?
The character, leader of Night Force, is another powerful DC Comics mystic.
Winters was destined to be the host for all the spirits not claimed by Heaven or Hell, which would drive him insane for eternity. He later escaped this fate by hiding out in a sort of Garden of Eden-like dimension, which makes sense because he had an existing, unspecified connection to Adam and Eve.
The asyulm where we see Constantine at the beginning of the episode is more or less the Hellblazer version of Arkham. It's one of a number of stories alluded to in the pilot that played out in the comics.
Yes, the Astra story played out similarly -- but not exactly the same -- in the comics. And, yes, it gets more complicated from here.
Dr. Roger Huntoon, the psychiatrist seen here, has worked for both Ravenscar and Arkham, and later in life develops a grudge on Constantine after John breaks up one of his relationships.
Ritchie Simpson who, in the comics, becomes a computer consciousness and then ends up as a techno demon. Here, he's surrounded by little toy robots at his desk.
At one point, we see "More Than This" written backwards on a wall. Are they teasing Zatanna, the backwards-talking DC Comics magician?
The demon Furcifer doesn't appear in the comics that we can find, but we will say those books of magic might be the Books of Magic, which would be interesting...!
There's a crossbow that oculd belong to Hawkgirl, someone from the Demon Knights, etc., and a bust that could be Ibis or Shazam
Eye of Horus
An Egyptian sun god, Horus in the comics is the one responsible for providing Black Adam and his sidekick Osiris with their powers. He was also one of the beings teased as a potential replacement for Doctor Fate when one of Fate's hosts died.
"I have a contact at Ivy University," Constantine says at one point.
That's an interesting place for them to name drop. Employees at Ivy have historically included, among others, Ray Palmer, The Atom, who is currently appearing on Arrow.
Who would walk with me?
Who would be crazy enough to walk with John Constantine? It seems the answer is Zed, one of his most frequent companions from the comics, who is seen at the end of the episode drawing all the crazy drawings of Constantine -- many of which either are, or closely resemble, famous images from Hellblazer comics.