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(Hoping this isn't another too-broad question...)
I just want a different way for my aliens to breathe that isn't obvious to a human's eyes; it makes them seem like they don't breathe at all, and their chests don't rise and fall with every inhale-exhale.
Double set of lungs:
imagine a creature much like a mammal. now you put a second chest in front of the fist one. Attach the muscles to the barrier between the front and the back lungs.
When the front lungs are expanded, the rear lungs are compressed, and vice versa. This way, the breathing apparatus is much like ours but doubled, but the motion happens internally. I'm not sure, but i think breathing might be more efficient too, because of the compression. Any variation on that theme (a lateral barrier between mammal-like lungs for example) might work, too.
If what you want is simply for the animals not to appear like they're breathing, you're in luck: the method you want already exists, and it mostly involves having rigid torsos.
Birds are nice examples, but not quite. While the lack a diaphragm like mammals do, their chests also move while they breathe, as muscles in their torso move the vertebrae and sternum up and down in order for it to expand in volume.
But there's one kind of animal that, despite not having a rigid exoskeleton doesn't quite enjoy the benefits of a more flexible torso: turtles.
Turtle shells, far from some exclusively external armor like you'd see in something like an armadillo, are composed of their ribs and vertebrae. Today we believe this came as an adaptation for digging, but regardless, the final outcome is a creature with a completely rigid torso that can't really expand to let in air.
So, how do they do it? Pretty simple, they transferred the job from muscle and bone to just muscle. Turtles have sheets of muscle specialized to contract and relax in order to force air in and out of the lungs inside their shells, allowing for the lungs to expand and contract inside fully rigid, unmoving torsos.
There's also another page from the turtle book that you can use in order to make your aliens seem like they don't breathe: butt breathing.
That's right, some turtles, especially some semiaquatic and aquatic species can exchange Gases through their butts,something known as cloacal respiration.
To explain it: breathing in the water,putting it simply, mostly takes 3 things: the water containing oxygen, a place full of blood vessels to get in contact with the water and get that oxygen through diffusion and a way to make that contact happen. Fish do this through their gills, but some turtles, already having a cloaca packed with blood vessels, adapted to use said cloacas to extract oxygen from the water, especially while hibernating underwater in the winter. This greatly helps turtles with a more aquatic lifestyle spend longer periods underwater, with some species being able to get all of the oxygen they need depending on how active they are at the moment.
So overall, if you want a species that doesn't look like they breathe, make them more turtle-like. A species with a rigid, armored torso housing their lungs, as well as muscle sheets responsible for handling their breathing. Couple that with the ability to extract oxygen from the water via something that isn't as obvious as gills and you could easily sell the idea of an alien species that simply does not breathe, or that does so through some overcomplicated, highly alien method, when you're actually just dealing with space turtles.
Scrotum frog style!
T.culeus breathes through its skin.
https://en.wikipedia.org/wiki/Telmatobius_culeus
The Titicaca water frog spends its entire life in oxygen-rich water that typically is 10–17 °C (50–60 °F). It mainly stays near the bottom and it does not surface to breathe if the water is well-oxygenated. It regularly performs "push-ups" to allow water to pass by its large skin folds, which absorb oxygen. It occurs even in deeper parts of Lake Titicaca, although the limit is unknown.
Your creatures also have redundant raisin like folds of skin, possibly all over like the scrotum frog, or possibly on a large pendulous respiratory organ. Instead of pushups, your aliens slowly dance, as though at all times they were doing tai-chi. In circumstances where more oxygen is needed the movements become more vigorous.
Bird lungs are unidirectional, so they don't breathe "in" and "out" quite like mammals. Their lungs don't need to constantly change size as much and they have higher oxygenation levels (yes, this is oversimplified). It does result in higher levels of inhaled particulates with less ability to cough things out (so birds have more respiratory infections).
Your species could use unidirectional flow respiration more like a pump or Chinese box bellows than a western-style bellows like our lungs. Air is constantly being drawn in at one spot and constantly expelled at another. With no net expansion of the lungs, there is no obvious motion. If the inhalation and exhalation locations are in less noticeable spots, then there would be almost no apparent breathing.
In Larry Niven's "Known Space" series there is a species of plant called "stage trees." They consist of a leafy cluster on top of a straight stem about 2 meters long and 20 cm thick.
The interior of the stem is essentially pure solid rocket fuel.
There are several cool stories about the origin of these plants, their growth patterns, etc. But the point is, they produce substantial quantities of chemical that includes both fuel and oxidant. Harvest the logs, dry them, strip off the bark, bundle them into a vehicle, and you've got something that will launch to space.
Imagine your folk have evolved to eat these. Floating about in their blood is a mixture of both the fuel and the oxidizer. And it comes in their food in exactly the right ratio to be 100 percent used up on reaction. No breathing required.
So the waste product is essentially water and CO2. Your folk would emit CO2 and water. They could emit the CO2 as gas through various small pores all over their bodies. And the water could be emitted as sweat or similarly to how we get rid of urine.
They would need some nifty chemicals in their blood so they were not explosively flamable. "Keep away from open flames." And their kitchens would be interesting. And No Smoking Please on their farms.
Perhaps they have somehow evolved a means to process and store oxygen so that it behaves more like ATP. So instead of breathing continuously they could have breathing "meals" where they intake a large amount of oxygen and store it for later use. I'm not sure how much oxygen you could store in something like perfluorocarbon, but it exists as an option for liquid breathing. Though I'm sure if pressed, evolution could come up with a better storage system.
That said, oxygen is not the only oxidizer, there are a lot of other alternatives for metabolic systems. Lithium ion batteries store a huge amount of energy without any oxygen required. It's not unfathomable that on a different planet the first life forms don't emit oxygen and therefore the Great Oxidation Event never occurs which in turn forces life to improvise an energy storage mechanism where oxygen isn't ubiquitously present. It would be less efficient to have to carry your oxidizer with you because it's usually heavier than the fuel, but if there's none of it conveniently in the air then that's what you'd have to do. For bonus story points that kind of life would have to deal with actual spontaneous combustion, as well as the occasional spontaneous detonation.
Hyper-porous lung dermis
Your species could have millions of tiny lungs all over their body, situated in their skin, each micro-lung having its own micro-diaphragm in the form of muscles similar to the ones that make your hair rise, the arrector pili muscles.
Imagine each and every pore on your body being able to imperceptibly take in air and exchange it with the blood throwing through your sub-dermal veins, in a vein similar to active osmosis if I had to put a term to it. Of course, just like with goosebumps you may be able to see what's going on if you look close enough, but from an arm's length or more(conversational distance) it's not going to be that obvious.
Air-Gills
Your aliens passively absorb oxygen from the air by using large gills. To make sure enough air passes over the gills, they need to be in constant motion (either by the whole alien moving or by the gills being moved through the air).
The answer about Stage Trees is good, but there are other possibilities.
Maybe they use an energy process not based on slowed-down combustion, and thus do not need a constant gas source as a combustion source.
Maybe they are plants, with their light exposure well balanced to their energy needs.
Maybe they are robots with rechargeable batteries.
Maybe their energy process is faster, but with a storage mechanism, so they breathe a lot while eating, but never need to after that.
