I have to say that both for work and for leisure I've had this situation come up often over 30+ years, and the only solution is black. If black doesn't suit your colouring, still go with black bottoms and shoes, because virtually any colour will go on top, including navy or brown (apart obvs from orange or yellow, unless you are a wasp or it's Halloween). Also, a black bag is discreet. My sole concession to luxury over, as I say, decades of compact travel is that now I sometimes take a black clutch with a crossover strap as well as the backpack for the flight. It helps, after all, even in the airport, for tickets and passports.
Take one bottom, one pair of shoes, and one bag (besides the actual backpack). You get on the plane in the single pair of trousers and pair of shoes. So, base on black jeans and black shoes. You could wear blue jeans, but they're not ideal for evenings or dinners in nice restaurants. Basically, in most of the world, dark jeans are formal enough for anything outside Buckingham Palace. (If you get invited to BP while on your weekend, there ARE shops outside the UK and pretty clothes are a great souvenir.)
Forget about joggers. Not ok anywhere unless you're at the gym or at bedtime. Europe isn't formal these days (I assume for a city break we're talking Europe, or the US) but it's not wearing joggers in the street. But your jeans can be skinny or bootleg or straight. (This assumes it's a short-haul flight, btw: for long-hauls, jeans can be uncomfortable.) You could, if you are not a jeans person, go with some other black trousers, though, from wide-leg to yoga pants to leggings to wool work-type trousers: as long as they're black, you can wear them from the airport to the cobbled roads to the Royal Opera House, I promise.
If you're expecting a lot of walking, trainers are the way to go unless you already have broken in some v v comfortable flats or ankle boots. But not WHITE trainers: not because they're out of fashion, though they are, but because they only work in daytime. Preferably even black trainers shouldn't have white soles but black soles, which are surprisingly difficult to find but can literally take you to the Palace.
Tops: me, I add cotton t-shirts for the flights, topped with a cardi in black or in my favourite colour for the flights (temperatures are v v unpredictable on planes and I also get surprisingly sweaty stressing about check-in and which gate) and for hotel/AirBnB rooms (one gets cold at night and room temps are variable). I don't mean granddad's droopy grey cardi, but a neat-fitting jumper that can unbutton at the front so it can be either jumper or jacket as you need.
For any evening event from dinner with DP to business dinner to opera at the New York Met, a pretty shirt or popover will gussy up your jeans. I have a black lace shirt that is my go-to, but also some very thin silk or polyester shirts that roll up small, don't wrinkle, and are pretty much ok for, again, the street, the cafe, the museum, the opera house. TBH even here at home, when in doubt I fall back on this combo.
For the most part, I find I don't want to stand out too much in foreign cities. If you expect to take a lot of photos for social media, though, consider taking a bright jacket. A true red is always great because it stands out against photos of buildings, etc, and suits almost any colouring. And the coat matters less in any formal situation because it comes off as you enter. At this time of year those pack-small Uniqlo type puffers are great. In almost all of the world, a navy blue or black puffer will work in any situation. Look around you in the UK: at this time of year everyone, but everyone, is in a darkish quilted puffer. But again, if you want great photos for your Instagram site, a red one will be perfect against the grayest weather or architecture, and I've noticed the Chinese photograph-everything women go for red. Or else consider going bright in your one cardigan or even scarf (I don't mean a silk square; I find a small warm solid-colour thin winter-type scarf a necessity though: again, variable temperatures)
TL;DR: stay with black, take only the one coat, trousers, shoes, and bag. Don't forget your toothbrush and a folding eco bag for expansion; if it looks like a shopping bag, most airlines will let it through on top of your backpack even if it's full of things you bought outside the airport. Also, bear in mind a lot of European AirBnBs don't supply shampoo, let alone hair conditioner. And if there's the slightest chance of a swimming pool, take your swimsuit: for difficulty of replacing at the last minute in a flattering style this could well be you most valuable item, even over an Hermes scarf.
Leave space in your school backpack to bring back your souvenirs, which in my case often include bright non-black unique clothes that I wear all over the UK!