As with most versions of the Fortress, it contains a whole lot of cool and mysterious things, including artifacts from alien worlds and "trophies" from Superman's previous adventures.
Most of it is pretty unrecognizable, likely either because artist Lee Weeks was just having fun with the backgrounds or because Weeks and writer Dan Jurgens have plans for some of it down the line. But there were a few things we could recognize.
You can see there that on the left there's a crossover SUV; we're guessing in spite of the coloring being off-model that's Lois and Clark's car, squirreled away after the events of last issue when it took some damage while fleeing Intergang.
In the center, there's the statue of Jor-El and Lara that adorns every version of the Fortress. They're holding aloft a globe of Krypton. It doesn't appear to be a perfect reproduction of any of the three versions of Krypton -- The Man of Steel, Birthright or Secret Origin -- that would have been in this Superman's memories, so maybe it's a sly merging of elements from all of them, or maybe even that Krypton's history was retroactively changed a little by the events of Flashpoint and/or Convergence.
Certainly the crystals everywhere seems like a call-out to Superman: Secret Origin, which used the visual iconography of the Richard Donner films as its touchstone and so had the crystal technology seen in those films (and in CBS's Supergirl, too).
There's also something that looks a lot like Superman's Kryptonian warsuit from the post-Crisis on Infinite Earths era. That suit showed up during a battle he had with the Eradicator and later was key to a powerless Superman finding his way back to Metropolis during the Reign of the Supermen.
Next, obviously, is Shockwave's armor. We saw Superman confiscate this in Superman: Lois and Clark #2, and it was one of the first things he felt he needed to "find a place for," since he felt like if he turned it into authorities it might just end up being used again.
Even taking villain escapes out of the equation, that's probably not bad thinking; Remember that in the pre-Flashpoint DC Universe Superman likely knows that Kate Spencer (Manhunter)'s entire origin revolved around stealing items from an evidence lockup to give her simulated powers.
You can put that axe into a big pile that says "vaguely familiar, but my brain isn't cooperating (along with the giant spaceship that's hovering overhead in the first panel), but take a look at that robot head on the far right.
...Doesn't that belong to the Iron Giant?! The coloring is different, obviously, but it sure looks familiar. Besides the overt Superman references in Iron Giant, that's kind of funny since Ron Marz and Doc Shaner recently did a story that was heavily inspired by that film for Adventures of Superman. Could this be a relic from that adventure?!
...Well, Bibbo was there, so it could very well be this Superman.
There's a laboratory where Superman is working with Hank Henshaw's unconscious form, but there's really nothing in there that's identifiable beyond Henshaw himself. Probably the last thing we see that's worth exploring is...
...The holding cells. Apparently this Superman has his version of The Pipeline from TV's The Flash, housing dangerous villains he's battled on site at the Fortress.0comments
Aside from Blanque and what looks like Stanley's Monster, I can't make out anyone too specific...but I have sinking feeling we'll be getting more acquainted with some of these characters soon.
You can pick up a copy of Superman: Lois and Clark #3 at your local comic shop or digitally on ComiXology here.