It might appear easy to add a quirk to your city.  Some people think they can just slap one on as they move along.  That can work at times, but there are moments where you have to think about what you’re doing.  After all, this set piece is supposed to catch the eye and become a symbol for the location.  There’s a lot more depending on it working out than one would initially believe.

1. Consider what the point of this quirk will be.  Is it something used for transportation or an entertainment area?  Maybe it’s a natural part of the region, which was there before the city.  It could be ruins.  In those last two cases, you’re looking at the set piece demonstrating a sense of history and time.  No matter what, you need a reason for a city to have this.  If it’s there simply to make the audience go ‘ooooh’ then it has a high risk of falling flat.
2. One source of inspiration can be the environment that the city is in.  For example, a place that is in arctic conditions may have something that keeps the place warm.  If you are looking at the environment then it is usually in a way that the locals have either defeated or utilized it.  This can be a big ‘man vs nature’ victory and show off the ingenuity of the people.  If this is a natural creation then it could be an oasis in the middle of a harsh landscape.
3. Another factor to consider is the level of tech and/or magic.  If this set piece needs to be created, you need it to match the tech and magic of the area.  They can’t be using elevators and teleporters when everything up until now has been medieval.  Not unless you establish that this city is an isolated location of technological advancement, but that brings in other challenges.  One way around this is to have the set piece be from ‘lost arts’ or something, but that means nobody can explain it or truly fix it if it breaks.
4. Sure . . . Go with some type of citywide tube system.  People don’t seem to have gotten bored of that yet.
5. A great way to explore this set piece is to have at least one character be unfamiliar with it.  That way, you have someone asking questions and possibly getting into trouble with the set piece.  This helps in two ways.  First, you can explain things to the audience in a more natural way than a blunt info dump.  Second, you force yourself to flush the set piece out and get a better idea of its purpose.  This allows you to have it be more than quirky background and can help you remember to have it be used throughout the course of the adventure.
6. Not every unique set piece needs to be a huge construct.  Sometimes, it can be a local creature or weather event.  There could be sturdy bubbles that can be jumped on and used a variety of ways like Sabaody in ‘One Piece’.  You can have hairy tapirs like in Curse of the Dark Wind.  This things still need a purpose and to be shown a few times, but they’re certainly not stationary such as towers, tube systems, and griffin aeries.
7. Nothing wrong with the set piece being small of size and large of scale.  We tend to think of these things as eye-catchingly gigantic.  Towers, arenas, mountains, waterfalls, and the list goes on.  Yet, you don’t need it to be big in order for it to achieve the same sense of awe and uniqueness.  Perhaps it is a single shrine that is incredibly important to the city.  This is harder to pull off because large things come with built in ‘wow’ factors, but that doesn’t mean it’s impossible.