For a first cruise any ship will be impressive. We absolutely loved our recent Grandeur cruise. Our 15 year old son had a great time and he's been on all the "wow" ships such as the Freedom, Liberty, and Mariner Of The Seas. They have the climbing wall, a teens-only "living room", the arcade, and pools. There is much to be said for smaller ships (Grandeur was considered a mega-liner when first introduced).
Want to know a popular thing among the teens? They'd congregate under the stairwells in small groups and talk. The kids will find a way to entertain themselves. Karaoke was a popular thing for the teens. A couple were quite talented.
The Grandeur was looking pretty good on our cruise. Food was amazingly good. It was significantly better than on our previous two or three RCI cruises. I'm not sure what happened - probably a change in the kitchen management - but we were very pleased. The staff was marvelous.