PJO HEADCANON: What I think the different cabins smell like
Listen, I love Percy. However he is a teenage boy whose left to his own devices half of the time. It smells kinda funky. Not terrible, faintly fishy/if you've ever been to the beach after the waves push a bunch of seaweed ashore and it bakes in the sun for a few hours. Percy bathes fairly regularly so it's not too much of a problem (it's at it's worst after camp activities for the day before he can shower), and he usually leaves the cabin windows open to air out the stale air.
Overall, not great but not awful
The Ares cabin smells BAD. This cabin I would say is definitely one of the most active ones with how much the Ares children are training and stuff. It's like a pungent B.O. smell. The worst part? The Ares kids know- they just don't care. In fact, some of them started taking a weird pride in it when during a training duel between an Ares kid and an Apollo kid, the Apollo kid made a taunt about their smell and in turn the Ares kid weaponized there stink a la throwing their sweaty towel at the kid. That event became an inside joke within the Ares cabin, and eventually it got out of hand when too many Ares kids were attaching sweaty shirts, socks, towels, etc. to their weapons during camp activities to get the upper hand (when Chiron asked why they kept doing this Clarisse told him it was "biological warfare").
You're telling me they're shoving 10+ prepubescent children into a barely 500 sq ft cabin during the height of a muggy humid Long Island summer and it's not going to smell funky?? Let me paint you a picture. Put a bunch of onions, garlic, and parmesan cheese into a bag. Take that bag and lock it in a sauna- max heat- for three months. I dare you to open that bag. Most of the Hermes kids don't even realize they smell because they've become so nose-blind to it. However as soon as they realize what's up they immediately begin plotting a variety of stink based pranks they can pull on the other cabins (The Stoll brothers got put on permanent stable mucking duty one summer for hiding a dirty sock in the Aphrodite cabin. They're cabin reeked for weeks and many of the campers had mental breakdowns when they couldn't find the source of the smell.)
Compared to some of the other cabins the Hephaestus cabin actually doesn't smell that bad. Since most of them are working by the forge for long hours at a time most of the Hephaestus kids have a pleasant smokey wood scent. The only time their smell turns sour is during the summer- all that protective gear makes them sweat A LOT, but unlike some cabins (...Ares...), the Hephaestus kids always bathe right after their done working for the day so it's never usually a problem.
The Demeter kids on average smell fairly neutral most of the time. I personally headcanon that how they smell is directly related to the weather and season.
(i.e, during the spring they usually smell very floral and bright, during the winter they smell crisp like pine or evergreens, before it rains they smell like ozone, etc.)
It actually became a fun camp tradition to guess the weather by asking one of the Demeter kids what they smell like (one time the Demeter kids were able to predict when a hurricane hit the coast of Long Island because the cabin REEKED like petrichor and ozone).
Zeus's cabin smells dusty. No one had been staying in that cabin for literal decade, when Thalia was staying there (pre and post turning into a tree) it smelled fine. Thalia never wore heavily scented perfumes or soaps, so if anything the cabin just smelled like nothing. However, after Thalia got turned into a tree and then later when she joins the hunters and the cabin is vacant- it was getting cleaned like what...once...twice a year? If you opened the door it would smell like walking into a mausoleum. Dusty, stale, with the faint lingering smell of something metallic)
Surprisingly I think the twins smell fine most of the time. It comes from a place of wanting to look good in front of their godly parent, and also there's not that many of them (I mean...it's just two guys), so any smell that might come off of them isn't building super heavy.
Dionysus also makes sure his kids are presentable in his own way. I mean- how would it reflect on him if his children were walking around smelling like ripe grapes and vinegar. He might make a light jab whenever he sees one of the boys looking rough around the edges and subtly uses his vines to lead the boys towards the showers.
The Apollo cabin is always 50/50 on whether or not they smell bad or not. These kids are artists, musicians, writers, poets- creatives! Any person who has a slightly creative passion will know that once you reach that flow state time melts around you and before you know it you accidentally stayed up the whole night working on one project. When one of the Apollo kids gets into this flow state it's not uncommon for one of them to accidentally forget to shower because their so caught up in getting the perspective in their painting just right, or trying to write the lyrics to the Campfire Night's next biggest hit. However, as soon as the Apollo kids get out of their flow state and realizes they're beginning to smell a bit musty- PANIC. Throw down whatever you're working on and absolutely book it to the showers for the world's longest everything shower.
The Aphrodite cabin is the only cabin that consistently smells amazing. Not only do the Aphrodite kids care a lot about self-grooming: daily showers (sometimes more than one a day), 10 step skin-care routines, hair oils, deodorants, etc. They love using luxury perfumes. Each Aphrodite kid has "their" specific scent, and arguments have broken out before when two campers chose perfumes that were too similar (these arguments never last long and usually by the end of the day the two campers at odds are sharing beauty tips and scent-profile advice like nothing ever happened). Chiron and Dionysus have been known to linger around the Aphrodite cabin when the other cabins begin smelling to funky (Chiron especially) to give their noses a break.
The Athena cabin smells like ASS. These little brainiacs- similar to the Apollo kids- get really really absorbed in their projects. Sometimes it's staying up and researching a specific historical figure, or battle formations, or weapons blue prints, etc. the only problem is that unlike the Apollo kids, the Athena kid grind set doesn't stop until they've leeched out all the information they can over whichever topic their brain decided to hyper-fixate on for that week. One time one of the Athena kids fixated on an extinct species of beetle and they spent 3 days deep diving every single piece of media they could on this insect. The average state of the Athena cabin is dark, stale pungent air, papers and books are strewn all over the place, with soda cans and energy bar wrappers all over the floors. You shoved a gaggle of ADHD riddled know-it-all geniuses into a confined space and you should expect nothing less than a library tornado in that cabin.
The longest record for an Athena kid so deep in the zone they forgot to take care of themself belongs to Annabeth. She had a dream, and in that dream she dreamt a blueprint. No one saw her for six days until Chiron went looking for her and found her hunched over her desk in the cabin scarfing down 5 hour energy drinks (no one knows where she got them from), had slept a total of 15 hours, and hadn't showered that whole time. There's no time to shower, the blueprint of my dreams demand to be materialized damn it!
The only time the Athena cabin (and kids) smell decent is whenever they're not trapped in the grasps of their pursuit of knowledge, or on cabin check days where they haul ass to look presentable so they don't get docked points.