Cruises tend to provide activities and events for teenagers though some of these are badly planned and executed. The activities planned can sometimes be awkward and not interesting for all age groups.
I would suggest bringing other activities such as a portable DVD player, DVDs, books or portable game systems (psp, nintendo ds).
A balcony is a wonderful idea because rooms can sometimes get stuffy and it's really nice to have somewhere private to sit and relax.
Especially if the weather is hot it's nice to have the balcony door open to air out the room
Have fun!