Hi Julia, A Grand Gathering at Disney World...how special!
You are sure to have a Magical time together. I think it's really sweet that you are trying to do something extra special for your nieces. Based on my teenage daughters Disney experiences, here are my top 3 suggestions for you.
1)
Dolphins in Depth - This exciting tour is rated tops in my teen daughters book. I wrote about her experience
HERE. One adult would need to do this with your nieces, and since you are the group Travel Planner I suggest that you volunteer ;)
2)
Sea Raycers - These fun boats can be rented at several locations at Disney. The minimum age requirement is 12, so all of the teens could do it together. There's something extra special about zooming across the waterways of Disney World.
3)
TrenD- Every teen girl I know loves a shopping spree. The coolest, hippest boutique is located at Downtown Disney. The store is called TrenD and the styles are SO cute. You could give the girls each a
Disney Gift Card and bring them to Downtown Disney to choose souvenirs.
Hope this helps and let me know if I can assist you further with your planning,
Kaylene