Two in the same fortress, both of them sadly never completed: A swimming pool and a fallout shelter.
The Swimming Pool:
This was actually the end result of another unsuccessful project, an experiment to see if dropping a whole cluster of magnetite down a z-level would break it up into bitesize, ready-to-smelt chunks without wastage. One z-level didn't do the trick, and trying again with another cluster and a few more levels would have taken so much digging that I'd have had all my miners up to Legendary anyway, so I dropped the idea. (Anyone know how far I'd have to drop it to pull it off, if it's possible at all?)
Anyway, I now found myself with an exceptionally large hole at the base of a vertical cliff. After experimenting with using it as a holding tank for my horde of dogs and puppies (cages as they currently work are a bit of an immersion-breaker for me) and finding it only made the FPS even worse, I turned my attention to the thought of turning it into an artificial lake.
Then I looked at the cliff, and thought to myself, "that would be a seriously badass way to train swimming in adventure mode, if only I could get right out over the water..."
It all kind of snowballed from there. First I cleared the bottom of stone and dug out a deep end, then I added a 'filtration system'; two pumps keeping a constant flow of water to and from the brook to stop it getting stagnant, with provision to drain it completely if something sank to the bottom. A couple of diving boards -two wooden floor tiles- were built into the side of the cliff face, with steps leading up beside them next to a 'pool ladder'. I would have built a junior diving platform over the shallow end, but circumstances intervened.
The Bunker: This one also came about by pure happenstance. I'd dug in from the top of a hill and started working my way down, and I pondered the best means of building a well inside the fortress, given my relative inexperience, for about five minutes before I said "screw pumps". *rimshot*
Instead, I sank a staircase down to the level of the brook before excavating a cistern. As an afterthought I dug a long corridor out the side of the hill to provide my fisherdwarves with a shortcut... and hit a magnetite cluster.
So now I have this huge empty cavern in the basement that's too far from everywhere else to be worth using as a stockpile, and every other mineral seam is already being turned into an art gallery so why not try something new?
It started out as a mere panic room, a place to have everyone congregate during sieges; a few beds, a dining area and a food/booze stockpile behind some reassuringly solid iron floodgates. But the fortress stayed small but prosperous (I set the population cap at sixty and got lucky with a couple of strange moods early on) and I found myself with nothing much else that needed doing, so I ended up with plans for nothing less than a self-sufficient fortress within a fortress. Its own farms, an indoor fishing area, a selection of workshops and stored raw materials for moods... I'd just got to the point of wondering if I could import enough lead for complete radiation shielding when a fatal flaw in my planning became obvious.
This bunker was what I'd been doing instead of training a proper military, and all I had was the six marksdwarves of the Fortress Guard. On the other hand, I had mining enabled for every single dwarf except woodcutters and a couple whose skills I needed round the clock, so I should have weathered the first siege without major trouble... if my weaponsmith hadn't withdrawn from society for a few weeks whilst he worked on a war hammer forged from solid gold, which could have bought out that year's dwarven caravan all by itself. Its creator was among the many, many casualties of the first invasion, at which point I lost heart and abandoned the moment he was decently buried.