Think gifts that either improve her daily life or solve a problem she actually has. High-quality stuff she'll use constantly works best: a really good kettle, nice coffee beans or tea, quality bedding, a solid kitchen knife, premium toiletries, a weighted blanket (folds up easily), or a nice robe. Experiential gifts also crush it - concert tickets, restaurant gift cards, or a massage voucher get her out of the apartment and don't take up any space. If she's into cooking or coffee, specialty items that are consumable or compact fit the bill perfectly.
For practical gifts that genuinely help with small living: a really sturdy over-the-door organizer, a good desk lamp that doesn't hog table space, or storage containers that actually nest. A nice throw blanket doubles as both comfort and a way to warm up without cranking heat. If she travels or ever leaves the apartment, a really good travel pillow or carry-on luggage could be useful. Subscription boxes to anything she enjoys (coffee, snacks, books, magazines) keep giving without cluttering - she uses them up and they're gone.
Skip anything decorative unless you know for sure it fits her style. Don't get furniture, wall art, plants that need maintenance, or gadgets that need a home. When in doubt, just ask her directly what would actually make her life easier right now. Most people living in tight spaces know exactly what they need and genuinely appreciate when someone just hands them a gift card to their favorite store so they can grab what works for them.