@nowifeorgf depends how familiar you are with them and budget. Ideally it’s something they’re interested in, like anime or if they’re into skating, ect. But if it’s a standard girl gift handbags and makeup would be the classic choice. I’d avoid clothes that may not fit or perfume because that is often a personal taste, jewellery is also generally an intimate thing only really bought by boyfriends or really close friends. Maybe something techish like earphones, portable speaker, instant print camera since tech stuff is universally useful. Since they’re probably doing exams right now something relaxing like soothing tea kit, hot springs gift card, hot water bottle, bath salts, ect stuff. Or maybe something school related like those fancy thermos - everyone seems to have their adult sippy cups these days. If nothing else a cinema gift card is a blank fun ticket for the holidays - this generation tends to like experiences over material stuff so things like escape room gift cards will do okay. If you want to make the gift cheeky/ memorable you can lead them on a clue hunt, maybe clues hidden in layers of wrapping paper or many little gifts concealed in the gift pile, or around the building.
hope that’s got the brain tickling some ideas. Wishing your niece (I’m guessing, birthday?) happy day