I am planning a 16th birthday for my daughter in March. I am looking at the following:
1) special tours thru Disney
2) ordering a special basket delivered from the FB group Ear for Each Other. They have people who will deliver charcutiere boards, breakfast baskets, dessert baskets and cakes.
3) Having a special photography session - booked thru either thru Disney and Ear for Each Other
4) Booking a makeover
Special cake! Either at a TS ADR or delivery to room. Disney cakes are so much fun and unless you have a 16 yo niece who doesn’t like cake, I think it’s a safe bet.
Also, her own Disney gift card. I have a slightly younger niece and I can’t quite keep up with exactly what she loves. I know her general likes, but I don’t see her quite often enough to know if a thing like this or like that is best. But she is happy to go into great detail with me and show me everything she picks out herself with the money.
If she is an ear wearer, maybe having a pair of those delivered to her either right before or at the start of the trip might be fun. Or maybe there’s a Loungefly backpack she could pick out and you could get her before the trip.
I think the photography session a PP mentioned might also be a hit if your niece is into that. I hated my photo taken at that age, but my niece would love it.
We belatedly celebrated DD's 16th at WDW and I got her a couple of gifts for the trip to give her on her actual birthday - a pair of Loungefly ears and a customized "I'm celebrating" sweet 16 birthday button that I got from Etsy.