I've always wondered what really lies beyond the walls. The most we've seen of the area outside is the Female Titan arc and the scene that played before the theme in episode 1 of the anime. Both times, we didn't get to see much of anything besides lots of trees.
What else is out there? Old remnants of villages existing before the titans? Other walled countries?
The Female Titan arc (57th expedition) DOES NOT take place beyond the walls. It takes place in the area between Wall Maria and Wall Rose, so anything we see there is just a part of the lost territory within the walls. This also includes when we first met Levi and his squad, because the 56th expedition took place from Trost since Maria had already fallen.
MAJOR SPOILERS FROM CHAPTER 85 AND 86 OF THE MANGA
As it turns out, the walls are on an island called Paradis, with a sizeable amount of land between the walls and the island's coast, but a mainland off the west coast of Paradis is home to humans (the Eldian and Marleyan ethnicities) and is not infested with Titans as Paradis is.
I am not an expert at all in the attack on titan franchise but there are several sources quora post about the walls & wiki about walls & theory about walls & wall titans about what the walls are, what lies beyond them and how they came to be.
I will summarize these under the spoiler tag below, BUT! I do recommend that you read the manga because it goes sooooooo much further than the anime and the question you asked is one of those questions that once answered turns into at least 10 more questions. I've included a source for reading online as well.
Happy Reading!
Spoilers:
In short the walls are actually crystallized wall titans who crystalized to protect humanity under the rule of the first king, King Reiss. Now when he did this and sealed the rest of humans inside the wall he erased their memories as to how this process worked because he believed that a world "ruled by titans" was true peace. The full story is that the king knew things about the titans that forced him to accept them as superior beings the information about the king and his vision are here. I'm 98% sure that this contains a direct answer to your question but going any further spoils the manga for me so I stopped here. Venture at your on peril
TLDR: There is a world out beyond the walls that has been forgotten for reasons listed in the spoiler
What is known from the Anime:
As far as the citizens inside these walls know:
What is known from the Manga:
SPOILERS! Please do not read if you haven't read the manga until chapter 70. I strongly recommend to read the Manga since it is one of the most beautifully written pieces of literature.
The walls are created by Titans. Yes, some Titans have a shell-hardening power that can create construction material. Turns out the walls are full of gigantic titans inside that hardened their skin to create such a defense against the world-outside-the-walls. These Titans were ordered/controlled by the Coordinator which is the highest Titan power (as known so far) and can be fully activated by the Royal family (Reiss). After creating the walls, the Coordinator erased all previous memory of the people inside these walls. The Coordinator has God-like powers and for some unknown reason, every successor of the family bloodline who inherited the Coordinator power, decided that the walls are what is best for humanity. Yet this evil that the Coordinator is trying to protect humanity from, remains unrevealed. Maybe it is a specific human race (or some of them). Because as we know so far, there have been genocides for special races like the Asians or Ackerman. But maybe the threat is not a living person (or race), maybe it is just a difference in ideology between what the Reiss family believes, and what other human races might believe is best use of the Titan power.
TLDR;
So far, we know that there are Titans outside the walls. There are people (like Grisha Jaeger, or Zeke, or Ymir) who wander who knows where. Armin's grandpa book speaks about the Ocean and all these natural differences from inside the walls. We yet do not know if there are villages or cities.
My subjective opinion
The Walls represent the biggest metaphor of the series. It is a reminiscent of the dominant us vs. them ideology in history/war/politics etc.
The Island of Paradis is on the country of Madagascar, which is then followed by Africa and the other continents, (Look at bottom of the page) http://attackontitan.wikia.com/wiki/Eldia. As it's said that Titans were created by The subjects of Ymir, outside of The Walls. As the population of Titans wiped out more than 99% of the current world population today, leaving us to 1 million, yes, their are Titans outside of The Walls, just stranded outside of Paradis.
There are even Titans still on the island but outside of the actual walls, before Wall Maria was knocked out by the Colossal Titan. Oh yeah, you also wanted to see if theirs anything else besides Titans. Of course. After humanity had shrunk off to the walls, the cities like New York, Tokyo, London, Sydney, Beijing, all gone. They have been in the wreck by the destruction of the Titans. Their is also still remains of forests, and Roman cities from the BCs.
Conclusion: Their are Titans beyond the regions of the walls, and outside the island of Paridis and off Globaly, but without human society with them. Shown that the amount of Titans outside The Walls is prodigious. The cities and towns of Earth is all wiped out, except for dead monuments.
THERE ARE SPOLIER SO BE WARNED BEFORE READING
So if you read chapter 85 of the manga Eren and some of the survey Corp go to his house’ basement. We see a picture of Grisha Yeager with 2 other people yes, Zeke, His son which is the Beast Titan, His 1st wife Dina Fritz. Then there are 3 books stating about his life .........beyond the walls. He states there is life beyond the walls. It tell about his life beyond the wall. So for you information, beyond the walls are not just titans, there is humanity. The king erased their memories of his people to think there is no life beyond the walls. Well Reiner, Bertolt, Annie, And someone else who got eaten by Ymir to go to Paradis to get the founding titan which is the coordinate. So there that is what is beyond the walls.
The walls are on an island called Paradis and all the people living on it like eren and everyone else are called Eldians. In the outside world the humans are living in peace (kindoff) there are no titans as only Eldians can be turned into titans by injecting them with titan spinal fluid. Marley is the nation where Reiner Berthodlt Annie Ymir and Zeke come from. Marley toss eldians which they infect with the titan spinal fluid to change to titans and attack the walls as a punishment for their crimes.
Unknown source (Prefer not to tell. ごめんなさい) The world outside is more advanced than theirs. Beyond the island of Paradis is where the rest of humanity is. The walls are meant to keep the people inside. When they ventured beyond the island, they learned that they were already left by those who are far more superior than their kind. Outside, there were planes, battleships, phones, technology. The hierarchy used fear to convince the people to stay inside. There are no titans outside the walls. The story were never true. It should be also noted that the attack began because of curiosity to explain the world outside. どうも
