Make an above-ground city entirely out of natural stone (no obsidian casting!)
Seconded. That actually has some tangible benefits: dwarves told to stay inside will actually stay inside if the buildings are Subterranean, and farming both inside and out can only be done on natural floors.
It also is a good design challenge because you must work with the natural layout of the area-- that, or dig down so deep and later expose it all by digging out the surrounding rock. Digging out the surrounding rock can be dangerous as it either involves lots and lots of ramping (and make sure you remove all ramps of an area before you dig down, or else you'll get the occasional dangling piece of ramp in the middle of nowhere!), or collapsing natural rock floors/ceilings down.
It also makes a ton of stone, or not if you're just removing dirt layers. Another challenge is that you can't really undo any part of it, since you can't reform natural floors and walls.
That is, unless cast obsidian can recreate subterranean areas. I'm really not sure, I expect that once an area has been exposed to the outside then it will never be subterranean, but I've never experimented with casting obsidian outdoors. If it could create subterranean areas, you could place a city out of cast obsidian that has the benefits of natural stone. Bonus points if you can find an area that is part acquifier and build the city on top of that, so that you can place fountains, lakes, and baths wherever you like with a bit of engineering.