That would really neat if we got to see the Magic in CC
As for suggestions for you niece, there isn't oodles to do when your b/w 18-21. I usually see a couple movies, go to see the shows, sit up on deck by the adult pool since it's a little quieter, maybe I'll go to one of the clubs later on, and of course eat

after all it is a cruise
Diversions has a couple different board games that I usually wind up playing with my mom and her BF, since we're all board game lovers. It's also nice b/c there's usually no one in there during the day, so it's quite and air-conditioned
We also do some of the family activities together. The 12 yr old in our group is a ping pong champ, so I'll play with him and be beat bad
Nassau, my mom and I spend as much time as we can shopping, while the boys head back to the ship after an hour

although we might go to atlantis this time
On CC we all stick to the family beach, do the banana boat, rent bikes, snorkel, stuff like that. We're not to keen on the adult beach, not enough to do, but that's just us. The grouper game pavilion(i think that's what it's called) is a must for us also
Ok, I'll stop now, I can see I'm starting to ramble
I wouldn't worry though, your niece will find stuff to do. Every time I get on the ship I always think I'll be bored, but by the end you've gotta drag me off b/c I had so much fun.
P.S. The Stack is geared a little more for younger teens anyway. The first time we went I was 17 so I was young enough to go there. It was ok, but I wasn't crazy about it. If I was 14 or 15 I would've loved it. Anyway, my point is she's not missing much, IMO.