What ship are you going on............some have more sports activities than others (like the big Royal Carribbean ships). I've only cruised Carnival and they have had enough to keep my boys busy, but they are not 18. At least you have two 18 year olds so they can hang out together until they meet other teens their age. If on Carnival, there will be the pool area, ping pong, small basketball court (on some ships), maybe a vollyball court and shuffleboard. If on the big Royal Carribean ships, add to that list rock climbing, ice skating, small golf course (have not been on RCCL.......I don't know if these areas get too busy to be worth it or not.) There's always the dance club at night.
We cruised 2 times in a quad room (4 berths); this time, we'll be like Darcy. Two inside rooms, next to each other. We've also only done 7 day cruises.......5 days is not enough, in my opinion. Our excursions focus on beach time and snorkeling.