Sustainable doesn't automatically have to mean less fun - often it's quite the opposite. Experience vouchers are a safe bet, but I'd get a bit more specific: instead of just "voucher for a workshop," you could research together what she's actually interested in. A climbing course, photography, pottery, cooking class - if she has a say in choosing it, she'll take it much more seriously.
With cosmetics and clothing, I'd honestly be careful. Fair trade labels are great in principle, but 15-year-olds notice right away if a shirt looks uncool, no matter how sustainably it was made. That's an easy way to miss the mark. Better: go shopping together and give her a budget for fair brands she picks out herself. Then she has real input and will actually wear it.
In my experience, the best sustainable gift is something she *does* rather than just owns. A subscription to a natural cosmetics box where she makes her own blends, a good knife and a cookbook with sustainable recipes, or a high-quality stainless steel water bottle she takes everywhere. Things she uses regularly and that become part of her everyday life - those stick around longer than random stuff.