Sure there's lots for teens to do aboard cruise ships (especially the newer ones) other than disco parties. (Most of the dedicated teen rooms are called discos however they just don't offer discos there all night, every night. Instead it's a gathering place for teens to "hang out," watch videos, play electronic games, and partake in group activities such as scavenger hunts. There are a lot of sports activities available to teens, such as teen pool volleyball, basketball, etc. On Royal Caribbean's Voyager and Explorer of the Seas there are in-line skating tracks and rock climbing walls too. Some ships, such as the Disney Cruise Line ships, have a teen coffee house rather than a disco, as their meeting spot. Disney also offers teens-only shore excursions as does Carnival Cruise Line on Alaska itineraries.
So there is plenty for teens to do. It seems most just like meeting others their age and having more freedom than at home to hang out together at night.
I suggest you read another teen's notes about her favorite aspects of cruising. See the message from Yanna on the Family Message Board, dated 6/7/01.
I hope this helps!
Luisa