get your metabolism running more. drink a lot more water in general (if you find yourself slightly dehydrated always, then don't worry, you won't have the gastric fortitude to overhydrate), and eat breakfast. don't cut out carbohydrates entirely, but do cut down on them. besides the calories, there's really not much for you in bread/rice/etc, and obviously there's nothing good from junk food. replace them with non-trans fats and protein. fatty stuff has a lot of other content present and generally tastes a lot richer, so not only is it less nutritionally stuffty but it's also gonna feel more filling than carbs of a similar caloric content. as for protein- well, it's protein. do not starve yourself. it will only harm you. when your body goes below a certain caloric intake, it enters a sort of starving-protection mode, where your metabolism just loving dies, and your body starts eating your muscles before it starts eating your fat. you can prevent this by exercising while starving yourself, but you'd still have to deal with the other nutrient shortages. aim for 1700ish calories daily as your absolute minimum, and disperse them between breakfast, lunch and dinner. don't eat a small breakfast, either. eat something elaborate.
anyway i know you said passively but even short exercises will help you. if you start doing short but intense resistance training, the sudden spike in energy demand temporarily spikes your metabolism. you know how if you push your muscles to the very limits, they feel super weak (sometimes almost unusable even) for a short while after? that's because they're literally depleted of energy. if you wait a few minutes, some of the waste will have been cleared and the muscles will have gotten more glucose to work with - the cardiovascular system is a wonderful thing. weakness that lasts more than a few minutes is due to the muscles themselves actually being worn out. after a couple hours - or sometimes days - the weakness will be accompanied by, and then replaced by, pain. this is your muscles growing via repairing microscopic damage done from the exercise. this soreness is a good thing, and something you should aim for. anyway, exercise isn't just about muscles or weight loss.
there's way more variables at play, and long story short, everything is improved from exercising - even your alertness. cardio is better for outright weightloss long term, but that's the stereotypical make-fatties-wheeze exercising that takes like an hour or more and is probably what you don't wanna do. it is objectively the best, though. you're literally spending way more energy, which is why the body demands so much more breathing
