They could do anything a family could do. Movies, the shows each night, sports and games on the deck, swimming in the family pools, aquaduck. They can do the family trivia games, bring their own games, swim together on CC.
We were just on the Wonder and one of the most disappointing things was that my 16 year old could only go to the workout room for 1 hour per day from 9-10 pm. This was supposed to be through Vibe, but she just went on her own because no one from Vibe ever wanted to go. However, we had late dinner and she usually came and ate something, then took off early. She supplemented this with running on deck 4, but she's an athlete and really missed working out with weights on her regular schedule.
There are plenty of of excursions they can all do together - snorkeling, stingrays, biking. Usually the 18 year olds can drive a boat, but I'm not sure if they can have a minor as a passenger. You'd also need to be creative in booking the staterooms since the 18 year olds can't be the 'adult' in the room with minors.
I do think they'd have plenty to do and lots of fun even without using the clubs. My 16 year old rarely went to the club on our first cruise, but when her sister joined her on our second cruise, and our table mates included a 16 year old, she went more often. She had just as much fun on the first cruise as the second, just different activities.
Nancy